[AST] Cleanup on getting the underlying decl of using-shdow decl.

This should be a NFC cleanup. It removes a unnecessary loop to get the underlying
decl, and add an assertion.

The underlying decl of a using-shadow decl is always the original declaration
has been brought into the scope, clang never builds a nested using-shadow
decl (see Sema::BuildUsingShadowDecl).

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D123422