Use ParseUnqualifiedId when parsing id-expressions. This eliminates

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@85904 91177308-0d34-0410-b5e6-96231b3b80d8