[CMake] Split LLVM_DEFINITIONS before adding them

add_definitions is meant to take multiple definitions in the form of
a parameter that is a list. LLVM_DEFINITIONS is a string variable
though. In practice, passing a string variable to add_definitions
mostly works, except for the case when it starts with not only a
plain -DFOO, but a define with a value, i.e. -DFOO=42. In the case
of LLVM_DEFINITIONS starting with `-DFOO=42 -DBAR`, cmake ends up
passing `-DFOO="42 -DBAR"` to the compiler, which isn't what was
intended.

See https://gitlab.kitware.com/cmake/cmake/-/issues/22162 for
discussions on the matter.

This matches the recently updated docs for how users of LLVM are
supposed to use LLVM_DEFINITIONS in CMake, see
llvm/llvm-project@a2a65a5.

Author: mstorsjo

CMakeLists.txt

@@ -27,7 +27,8 @@ if(LLVM_BUILD_MAIN_SRC_DIR)
   include_directories(${LLVM_BUILD_BINARY_DIR}/tools/lldb/include)
 endif()
 
-add_definitions(${LLVM_DEFINITIONS})
+separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
+add_definitions(${LLVM_DEFINITIONS_LIST})
 
 if (NOT LLVM_ENABLE_EH)
   if(LLVM_COMPILER_IS_GCC_COMPATIBLE)