[Clang][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 Clang's CMakeLists.txt.

Get rid of the list(REMOVE CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
later, as list(REMOVE) is documented to remove *all* occurences of the
item, not just the one that was just added.  Instead, make use of
cmake_push_check_state() and cmake_pop_check_state(), which is already
used in other cmake files.

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)
@@ -183,18 +187,17 @@ check_include_file(sys/resource.h CLANG_HAVE_RLIMITS)
 # This check requires _GNU_SOURCE on linux
 check_include_file(dlfcn.h CLANG_HAVE_DLFCN_H)
 if( CLANG_HAVE_DLFCN_H )
+  include(CMakePushCheckState)
   include(CheckLibraryExists)
   include(CheckSymbolExists)
   check_library_exists(dl dlopen "" HAVE_LIBDL)
+  cmake_push_check_state()
   if( HAVE_LIBDL )
     list(APPEND CMAKE_REQUIRED_LIBRARIES dl)
   endif()
   list(APPEND CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
   check_symbol_exists(dladdr dlfcn.h CLANG_HAVE_DLADDR)
-  list(REMOVE_ITEM CMAKE_REQUIRED_DEFINITIONS -D_GNU_SOURCE)
-  if( HAVE_LIBDL )
-    list(REMOVE_ITEM CMAKE_REQUIRED_LIBRARIES dl)
-  endif()
+  cmake_pop_check_state()
 endif()
 
 set(CLANG_RESOURCE_DIR "" CACHE STRING