[CMake] LLVM_PROFDATA_FILE only works if you're using clang, so we should...

[CMake] LLVM_PROFDATA_FILE only works if you're using clang, so we should error out if it is specified when not using clang.

Also updated the CMake docs.

Based on post-commit review of r250108 from Sean Silvas.

llvm-svn: 250150