Simplify getTypeInfoImpl handling of 'non-canonical unless dependent' types.

These types are not dependent in this context, so just look through
the sugar.

Review by Richard Smith & Eli Friedman.
Correctly classify pack expansions as NON_CANONICAL_UNLESS_DEPENDENT

Test coverage for non-dependent pack expansions doesn't demonstrate a
failure prior to this patch (a follow-up commit improving debug info
will cover this commit specifically) but covers a related hole in our
test coverage.

Reviewed by Richard Smith & Eli Friedman.
PR16214, PR14467: DebugInfo: use "RequireCompleteType" to decide when to emit the full definition of a type in -flimit-debug-info

This simplifies the core benefit of -flimit-debug-info by taking a more
systematic approach to avoid emitting debug info definitions for types
that only require declarations. The previous ad-hoc approach (3 cases
removed in this patch) had many holes.

The general approach (adding a bit to TagDecl and callback through
ASTConsumer) has been discussed with Richard Smith - though always open
to revision.