Correctly handle using foo = std::foo inside namespaces.

Summary:
The gdb pretty printer misprints variables declared via
using declarations of the form:

namespace foo {
using string_view = std::string_view;

string_view bar;
}

This change fixes that, by deferring the decision to ignore
types not inside std until after desugaring.

Reviewers: #libc!

Subscribers: broadwaylamb, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D76816