GH58368: Correct concept checking in a lambda defined in concept

As that bug reports, the problem here is that the lambda's
'context-decl' was not set to the concept, and the lambda picked up
template arguments from the concept.  SO, we failed to get the correct
template arguments in SemaTemplateInstantiate.

However, a Concept Specialization is NOT a decl, its an expression, so
we weren't able to put the concept in the decl tree like we needed.
This patch introduces a ConceptSpecializationDecl, which is the smallest
type possible to use for this purpose, containing only the template
arguments.

The net memory impliciation of this is turning a
trailing-objects into a pointer to a type with trailing-objects,  so it
should be minor.

As future work, we may consider giving this type more responsibility, or
figuring out how to better merge duplicates, but as this is just a
template-argument collection at the moment, there isn't much value to
it.

Differential Revision: https://reviews.llvm.org/D136451