Make the -Wkeyword-compat diag message more accurate

Changed from:

  keyword '__is_empty' will be treated as an identifier for the remainder of the translation unit

To:

  keyword '__is_empty' will be made available as an identifier for the remainder of the translation unit

This is a more accurate description of clang's keyword compatibility feature,
given that some of the keywords are turned into context-sensitive keywords
(e.g. REVERTIBLE_TYPE_TRAIT) rather than being fully disabled.

llvm-svn: 196776