[libc++] Do not force going through xcrun to find Clang in the macOS CI scripts

It should be possible to run those CI scripts with different compilers
by simply exporting a different CXX environment variable.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356562 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed