[OpenMP][amdgcn] Don't use in-tree clang if not available.

The use of `$<TARGET_FILE:clang>` was adapted too broadly from D101265.

Fixes llvm.org/PR51579

Also see discussion in D108534.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D108640

GitOrigin-RevId: 1275ee3041049c69e9aaa7ac76af83a70340e054
1 file changed