[clang] Preserve Qualifiers and type sugar in TemplateNames

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.