[Clang][Sema] Fix comparison of constraint expressions

This diff switches the approach to comparison of constraint expressions
to the new one based on template args substitution.
It continues the effort to fix our handling of out-of-line definitions
of constrained templates.

The associated GitHub issue: https://github.com/llvm/llvm-project/issues/61414

Test plan:
1/ ninja check-all
2/ bootstrapped Clang passes tests

Differential revision: https://reviews.llvm.org/D146178