[clang] [compiler-rt] [llvm] [CMake] Use Clang to infer the target triple (PR #89425)

[YunQiang Su via cfe-commits]

================
@@ -28,6 +28,19 @@ if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   if (NOT LLVM_RUNTIMES_BUILD)
     load_llvm_config()
   endif()
+  if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
----------------
wzssyqa wrote:

With my patch, `libclang_rt.builtin` has been installed to per_target runtime dir.
I don't think that this one is needed.

And `if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` is not needed here: it is just set the triple.
If `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=off`, the libraries will still install to non-per-target dir.

And `if(NOT APPLE)` is not needed either.
```
$ ./bin/clang --target=air64-apple-ios16.0 -print-target-triple  
air64-apple-ios16.0
$ ./bin/clang --target=arm64-apple-darwin23.4.0 -print-target-triple  
arm64-apple-darwin23.4.0
```

https://github.com/llvm/llvm-project/pull/89425