We've been improving these the tests for vector quite a bit and we are
probably not done improving our container tests. Formatting everything
at once will make subsequent reviews easier.

This patch improves the preservation of qualifiers and loss of type
sugar in TemplateNames.

This problem is analogous to https://reviews.llvm.org/D112374 and this
patch takes a very similar approach to that patch, except the impact
here is much lesser.

When a TemplateName was written bare, without qualifications, we
wouldn't produce a QualifiedTemplate which could be used to disambiguate
it from a Canonical TemplateName. This had effects in the TemplateName
printer, which had workarounds to deal with this, and wouldn't print the
TemplateName as-written in most situations.

There are also some related fixes to help preserve this type sugar along
the way into diagnostics, so that this patch can be properly tested.

- Fix dropping the template keyword.
- Fix type deduction to preserve sugar in TST TemplateNames.

I made sure they all had some expected-error output in them. Many of
these tests would be better implemented as a positive test using SFINAE,
but this is beyond the scope of this patch.

Differential Revision: https://reviews.llvm.org/D153980