[AST] Use RecoveryExpr to model a DeclRefExpr which refers to an invalid Decl.

Previously, we didin't build a DeclRefExpr which refers to an invalid declaration.

In this patch, we handle this case by building an empty RecoveryExpr,
which will preserve more broken code (AST parent nodes that contain the
RecoveryExpr is preserved in the AST).

Differential Revision: https://reviews.llvm.org/D120812