[clang] Implement sugared substitution changes to infrastructure

Implements the changes required to perform substitution with
non-canonical template arguments, and to 'finalize' them
by not placing 'Subst' nodes.

A finalized substitution means we won't resugar them later,
because these templates themselves were eagerly substituted
with the intended arguments at the point of use. We may still
resugar other templates used within those, though.

This patch does not actually implement any uses of this
functionality, those will be added in subsequent patches,
so expect no changes to existing tests.

Signed-off-by: Matheus Izvekov <mizvekov@gmail.com>

Differential Revision: https://reviews.llvm.org/D134604