[AST] lookup in parent DeclContext for transparent DeclContext

The compiler would crash if we lookup for name in transparent decl
context. See the tests attached for example.

I think this should make sense since the member declared in transparent
DeclContext are semantically defined in the enclosing (non-transparent)
DeclContext, this is the definition for transparent DeclContext.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D116792