Integrate Hermes with the Xcode build process

Summary:
## Context
If React Native is built from *main* of any non-stable commit, then Hermes is built from source. The build is performed by `build-ios-framework.sh` and `build-mac-framework.sh` scripts in `hermes-engine.podspec` `prepare_command` stage. Since those scripts have no access build target information, they build all possible architectures and platforms just in case. That takes ages.
## Solution
The idea is to integrate build script into Xcode *run script* phase, and use build target information to build Hermes for active architecture only.
## Implementation
- Existing behaviour remains unchanged for local tarball and remote prebuild cases.
- `build-hermesc-xcode.sh` builds Hermesc as `hermes-engine.podspec` `prepare_command`. Default build location is `react-native/sdks/hermes-engine/build_host_hermesc`.
- `build-hermes-xcode.sh` builds Hermes in 'Build Hermes' Xcode script phase. It uses `$PLATFORM_NAME`, `$CONFIGURATION`, `$ARCHS`, `$IPHONEOS_DEPLOYMENT_TARGET` and `$MACOSX_DEPLOYMENT_TARGET` environment variables to configure cmake project so it builds only active architecture. The script also gets RN version, *cmake* path and *hermesc* path from the podspec.
- `copy-hermes-xcode.sh` copies Hermes.framework inside the app bundle. This script phase is added to the user app target in a `post_install` hook, after pods are integrated in a user project.
- `OTHER_LDFLAGS -framework "hermes"` added to the user project to enable linking against Hermes.framework.
- If `HERMES_OVERRIDE_HERMESC_PATH` is set, then Hermesc building is skipped, and `HERMES_OVERRIDE_HERMESC_PATH` is used for `build-hermes-xcode.sh`.
- `HERMES_CLI_PATH` is injected into user project config to enable Hermes source maps in `react-native-xcode.sh`.
## Things that didn't work
- *Running build-hermesc-xcode.sh in Xcode run script phase*. This doesn't work because Hermesc is supposed to be built for macos, and if build target is ios, then Xcode configures environment in such a way that Hermesc build fails.
- *Installing Hermesc into CocoaPods download folder*. So it then ends up in `Pods/hermes-engine/build_host_hermesc`, and all the housekeeping is handled by CocoaPods. This doesn't work because cmake uses absolute paths in a configured project. If configured project is moved to a different location, nothing builds.
- *Installing Hermesc directly into Pods/hermes-engine*. This doesn't work because CocoaPods runs prepare_command before Pods folder clean up, and everything gets wiped.
## Known issue
-  If `Pods/hermes-engine` is manually removed, then `sdks/hermes-engine/build_host_hermesc` must also be removed before running `pod install`. Otherwise cmake will complain about stale cache:
```
CMake Error: The source "<CocoaPodsCache>/hermes-engine/<hash2>/CMakeLists.txt" does not match the source
"<CocoaPodsCache>/hermes-engine/<has1>/CMakeLists.txt" used to generate cache.  Re-run cmake with a different source directory.
```
## Benchmark
MacBook M1 2021 32 GB.
```
export REACT_NATIVE_PATH=~/fbsource/xplat/js/react-native-github
cd $REACT_NATIVE_PATH/packages/rn-tester
pod install
rm -rf $REACT_NATIVE_PATH/sdks/hermes-engine/build_host_hermesc
cd $REACT_NATIVE_PATH/packages/rn-tester/Pods/hermes-engine
echo 't1=$(date +%s); $@; t2=$(date +%s); diff=$(echo "$t2 - $t1" | bc); echo Operation took $diff seconds.' > /tmp/benchmark.sh
```
```
# Before
export BUILD_TYPE=Debug
export JSI_PATH=$REACT_NATIVE_PATH/ReactCommon/jsi
export RELEASE_VERSION=1000.0
export IOS_DEPLOYMENT_TARGET=iphonesimulator
export MAC_DEPLOYMENT_TARGET=12.6
cd $REACT_NATIVE_PATH/packages/rn-tester/Pods/hermes-engine
. /tmp/benchmark.sh $REACT_NATIVE_PATH/sdks/hermes-engine/utils/build-ios-framework.sh
# Operation took 252 seconds
. /tmp/benchmark.sh $REACT_NATIVE_PATH/sdks/hermes-engine/utils/build-mac-framework.sh
# Operation took 179 seconds
```
```
# After
. /tmp/benchmark.sh source $REACT_NATIVE_PATH/sdks/hermes-engine/utils/build-hermesc-xcode.sh $REACT_NATIVE_PATH/sdks/hermes-engine/build_host_hermesc
# Operation took 59 seconds.
. /tmp/benchmark.sh xcodebuild -workspace $REACT_NATIVE_PATH/packages/rn-tester/RNTesterPods.xcworkspace -scheme hermes-engine
# Operation took 106 seconds.
```
|Before|||After|||
|--|
|iOS framework (s)|Mac framework (s)|Total (s)|Hermesc (s)|Target-specific framework (s)|Total (s)|
|252|179|431|59|106|**165 (-266) (-61%)**|
The performance win is fixed, and does not depend on the project size and structure.
As an example, this is how these changes affect build time of RNTester.
|Before||||After|||
|--|
||Pod install (s)|Xcode build (s)|Total (s)|Pod install (s)|Xcode build (s)|Total (s)|
|Clean build|1219|132|1352|734 (-485)|249(+117)|**983 (-369)**|
|Incremental build|82|30|112|105 (+23)|**34 (+4)**|139 (+27)|
The most important values here are the total clean build time and the incremental Xcode build time. The first one went down by 369 seconds, the second one went up by 4 seconds. I consider it a reasonable tradeoff.
The extra 4 seconds in  the incremental Xcode build time can potentially be mitigated by setting up output file lists for the new script phases.

allow-large-files

Changelog:
[iOS][Changed] - Hermes is integrated into Xcode build.

Reviewed By: hramos

Differential Revision: D40063686

fbshipit-source-id: e6993d62225789377db769244bc07786cc978a27