[AST][RecoveryExpr] Popagate the error-bit from a VarDecl's initializer to DeclRefExpr.

The error-bit was missing, if a DeclRefExpr (which refers to a VarDecl
with a contains-errors initializer).

It could cause different violations in clang -- the DeclRefExpr is value-dependent,
but not contains-errors, `ABC<DeclRefExpr>` could produce a non-error
and non-dependent type in non-template context, which will lead to
crashes in constexpr evaluation.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D86048