As threatened previously: consolidate name lookup and the creation of

DeclRefExprs and BlockDeclRefExprs into a single function
Sema::ActOnDeclarationNameExpr, eliminating a bunch of duplicate
lookup-name-and-check-the-result code.

Note that we still have the three parser entry points for identifiers,
operator-function-ids, and conversion-function-ids, since the parser
doesn't (and shouldn't) know about DeclarationNames. This is a Good
Thing (TM), and there will be more entrypoints coming (e.g., for C++
pseudo-destructor expressions).

llvm-svn: 59527