Update LLVM submodule

This commit updates the LLVM submodule to the current trunk of LLVM itself. This
brings a few notable improvements for the wasm target:

* Support for wasm atomic instructions is greatly improved
* Renamed memory wasm intrinsics are fully supported
* LLD has fixed a quadratic execution bug with large numbers of relocations in
  wasm files.

The compiler-rt submodule has been updated in tandem as well.