Refactor ActOnDeclarationNameExpr into a "parsing action" part and a

"semantic analysis" part. Use the "semantic analysis" part when
performing template instantiation on a DeclRefExpr, rather than an ad
hoc list of rules to construct DeclRefExprs from the instantiation.

A test case for this change will come in with a large commit, which
illustrates what I was actually trying to work on.

llvm-svn: 74528