Revert r273898's UNICODE quick fix in favor of CMake's remove_definitions()

UNICODE and _UNICODE defintions were added in the LLVM CMake build system.
While on Unices, the UNICODE/_UNICODE macros don't cause problems, on Windows
only ittnotify_static.c should be compiled using -DUNICODE.  We are still
looking at a proper fix, but this change sets the build back to exactly what it
was doing before.  Also, a comment and TODO were added in the src/CMakeLists.txt
file to help explain.

llvm-svn: 274052