[Clang][Sema] Start fixing handling of out-of-line definitions of constrained templates

This diff starts fixing our handling of out-of-line definitions of constrained templates.
Initially it was motivated by https://github.com/llvm/llvm-project/issues/49620 and
https://github.com/llvm/llvm-project/issues/60231.
In particular, this diff adjusts Sema::computeDeclContext to work properly in the case of
constrained template parameters.

Test plan:
1/ ninja check-all
2/ Bootstrapped Clang passes all the tests
3/ Internal testing

Differential revision: https://reviews.llvm.org/D145034