Use ParseUnqualifiedId when parsing id-expressions. This eliminates

yet another copy of the unqualified-id parsing code.

Also, use UnqualifiedId to simplify the Action interface for building
id-expressions. ActOnIdentifierExpr, ActOnCXXOperatorFunctionIdExpr,
ActOnCXXConversionFunctionExpr, and ActOnTemplateIdExpr have all been
removed in favor of the new ActOnIdExpression action.

llvm-svn: 85904