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

[Petr Hosek via cfe-commits]

petrhosek wrote:

This is a re-upload of https://reviews.llvm.org/D140925, I plan to land this change if there are no objections.

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