[clang] [Clang] Set the FTM for trivial relocation (PR #142936)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> @cor3ntin I think Apple Clang still complicates version detection, and we can't use __has_builtin, because that's returned true for versions of Clang where it's "broken" from libc++'s perspective.

`__builtin_is_cpp_trivially_relocatable` is new in 21, it was never shipped, nor is it "broken", so it should be a suitable
test



https://github.com/llvm/llvm-project/pull/142936