[AST] Better recovery on an expression refers to an invalid decl.

Prior to the patch, we didn't build a DeclRefExpr if the Decl being
referred to is invalid, because many clang downstream AST consumers
assume it, violating it will cause many diagnostic regressions.

With this patch, we build a DeclRefExpr enven for an invalid decl (when the
AcceptInvalidDecl is true), and wrap it with a dependent-type
RecoveryExpr (to prevent follow-up semantic analysis, and diagnostic
regressions).

This is a revised version of https://reviews.llvm.org/D76831

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D121599