[Concepts] Fix Concepts on generic lambda in a VarTemplateSpecDecl

[Concepts] Fix Concepts on generic lambda in a VarTemplateSpecDecl

As fallout of the Deferred Concept Instantiation patch (babdef27c5), we
got a number of reports of a regression, where we asserted when
instantiating a constraint on a generic lambda inside of a variable
template. See: https://github.com/llvm/llvm-project/issues/57958

The problem was that getTemplateInstantiationArgs function only walked
up declaration contexts, and missed that this is not necessarily the
case with a lambda (which can ALSO be in a separate context).

This patch refactors the getTemplateInstantiationArgs function in a way
that is hopefully more readable, and fixes the problem with the concepts
on a generic lambda.

Differential Revision: https://reviews.llvm.org/D134874