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

[via cfe-commits]

jeremyd2019 wrote:

> I agree that the second change, that is using `LLVMConfig.cmake`'s `LLVM_DEFINITIONS`, should be applied to other projects that support standalone builds like LLD.

Is there a list somewhere or do I just rely on my grep-foo?
* bolt
* clang
* flang
* lld
* lldb
* mlir

grep also turned up these, but they don't seem like things that link to LLVM, so probably shouldn't have these defines.
* compiler-rt
* libc
* libclc
* llvm-libgcc



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