Fix compiler extension in standalone mode

Use a dedicated cmake file to store the extension configured within LLVM. That
way, a standalone build of clang can load this cmake file and get all the
configured standalone extensions.

This patch is related to https://reviews.llvm.org/D74602

Differential Revision: https://reviews.llvm.org/D74757

GitOrigin-RevId: 3a0f6e699bb6d96dc62dce6faef20ac26cf103fd
1 file changed