Simplify RevertibleTypeTraits as a form of contextual keyword

Now that we emit diagnostics for keyword-as-identifier hacks (-Wkeyword-compat)
we can go ahead and simplify some of the old revertible keyword support.

This commit adds a TryIdentKeywordUpgrade() function to mirror the recently
added TryKeywordIdentFallback() and uses it to replace the hard-coded list of
REVERTIBLE_TYPE_TRAITs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197496 91177308-0d34-0410-b5e6-96231b3b80d8