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.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@196776 91177308-0d34-0410-b5e6-96231b3b80d8