[clang] [Clang][CMake] respect LLVMConfig.cmake's LLVM_DEFINITIONS (PR #138587)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> If this is right, it should probably be done to other standalone-capable projects' CMakeLists.txt also (LLD in particular, for my interests). Actually, it seems there's nothing in LLD that requires _GNU_SOURCE on Cygwin...

Yep, indeed.

I guess the main question is who others regularly build in standalone mode and is familiar with how this is supposed to work, cmake-wise. CC @mgorny @tstellar who seem to have touched things for standalone build mode, and CC @petrhosek for overall cmake review.

https://github.com/llvm/llvm-project/pull/138587