[CMake] respect LLVMConfig.cmake's LLVM_DEFINITIONS in standalone builds (#138587)

In #138329, _GNU_SOURCE was added for Cygwin, but when building Clang
standalone against an installed LLVM this definition was not picked up,
resulting in undefined strnlen. Follow the documentation in
https://llvm.org/docs/CMake.html#embedding-llvm-in-your-project and add
the LLVM_DEFINITIONS in standalone projects' cmakes.

GitOrigin-RevId: ba4bd3f46e97b5637b16a0fa74b064fb3e6db8ff