[CMake] respect LLVMConfig.cmake's LLVM_DEFINITIONS

In llvm#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#developing-llvm-passes-out-of-source
and add the LLVM_DEFINITIONS in standalone projects' cmakes.

Author: jeremyd2019

clang/CMakeLists.txt

@@ -68,6 +68,10 @@ if(CLANG_BUILT_STANDALONE)
   option(CLANG_ENABLE_BOOTSTRAP "Generate the clang bootstrap target" OFF)
   option(LLVM_ENABLE_LIBXML2 "Use libxml2 if available." ON)
 
+  separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
+  add_definitions(${LLVM_DEFINITIONS_LIST})
+  list(APPEND CMAKE_REQUIRED_DEFINITIONS ${LLVM_DEFINITIONS_LIST})
+
   include(AddLLVM)
   include(TableGen)
   include(HandleLLVMOptions)