[AST] Dont invalidate a ref-type var decl if it has no initializer.

This would allow more AST nodes being preserved for broken code, and
have a more consistent valid bit for ref-type var decl (currently, a
ref-type var decl with a broken initializer is valid).

Per https://reviews.llvm.org/D76831#1973053, the initializer of a variable
should play no part in its "invalid" bit.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D122935