Make Selector::getIdentifierInfo() private. I took a first attempt at this last

week in:

http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090302/013580.html

That patch caused the output of the diagnostics to change. Since
'DeclarationName' can already reason about Selectors and the Diagnostics logic
reasons about DeclarationName, there is no additional code needed to get the
diagnostics working by making Selector::getIdentifierInfo() private.

llvm-svn: 66992