Fix infinite recursion in deferred diag emitter

Currently deferred diagnostic emitter checks variable decl in DeclRefExpr, which
causes infinite recursion for cases like long a = (long)&a;.

Deferred diagnostic emitter does not need check variable decls in DeclRefExpr
since reference of a variable does not cause emission of functions directly or
indirectly. Therefore there is no need to check variable decls in DeclRefExpr.

Differential Revision: https://reviews.llvm.org/D76937