[docs] [CMake] Change recommendations for how to use LLVM_DEFINITIONS

[mstorsjo]

Summary

LLVM_DEFINITIONS is a string variable containing a list of arguments
to pass to the compiler. When CMake's add_definitions is passed a
string variable, this is interpreted as one argument. To make it
behave properly, the string variable needs to be split into a list.

Despite the fact that add_definitions isn't supposed to be used like
the LLVM docs recommended, it worked fine in practice in many cases.
If the first argument in LLVM_DEFINITIONS is of the form -DFOO=42
instead of plain -DFOO, the rest of the string is treated as value
to this define. I.e. if LLVM_DEFINITIONS consists of -DFOO=42 -DBAR,
CMake ended up passing -DFOO="42 -DBAR" to the compiler.

See https://gitlab.kitware.com/cmake/cmakissues/22162
for discussion on the matter.

Changing LLVM_DEFINITIONS to be a list variable would possibly be
more disruptive; instead keep the variable defined as before but
change the recommendation for how to use it. Then projects using it
can gradually be updated to follow the new recommendation.

[phosek]

LGTM

llvm/docs/CMake.rst

.. code-block:: cmake
   message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 
   # Set your project compile flags.
   # E.g. if using the C++ header files
   # you will need to enable C++11 support
   # for your compiler.
 
   include_directories(${LLVM_INCLUDE_DIRS})
-  add_definitions(${LLVM_DEFINITIONS})
+  separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
+  add_definitions(${LLVM_DEFINITIONS_LIST})
 
   # Now build our tools
   add_executable(simple-tool tool.cpp)
 
   # Find the libraries that correspond to the LLVM components
   # that we wish to use
   llvm_map_components_to_libnames(llvm_libs support core irreader)
 
<project dir>/
           ...
 
 Contents of ``<project dir>/CMakeLists.txt``:
 
 .. code-block:: cmake
 
   find_package(LLVM REQUIRED CONFIG)
 
-  add_definitions(${LLVM_DEFINITIONS})
+  separate_arguments(LLVM_DEFINITIONS_LIST NATIVE_COMMAND ${LLVM_DEFINITIONS})
+  add_definitions(${LLVM_DEFINITIONS_LIST})
   include_directories(${LLVM_INCLUDE_DIRS})
 
   add_subdirectory(<pass name>)
 
 Contents of ``<project dir>/<pass name>/CMakeLists.txt``:
 
 .. code-block:: cmake