Fix macOS fuzz: use Xcode clang instead of Homebrew LLVM

Homebrew LLVM 18's ASAN/libFuzzer runtimes contain weak-def symbols
for typed allocation operators (__ZnwmSt19__type_descriptor_t) that
don't exist in macOS 14's system libc++. Since the symbol is embedded
in the pre-built runtime dylibs (not our code), link-time flags cannot
fix it.

Switch to Apple's Xcode clang which ships its own libFuzzer and ASAN
runtime built against the system libc++ — no ABI mismatch possible.
No Homebrew LLVM install needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>