[clang] [Clang][Sema] Fix an assertion crash when instantiating a nested requirement (PR #213660)

[via cfe-commits]

babadany2999 wrote:

Thank you @zyn0217 for the review.

I've followed your instructions and:
- Moved the testing unit into an existing testing file (`clang/test/CXX/expr/expr.prim/expr.prim.req/nested-requirement.cpp`). I've run ```grep -R "nested requirement" clang/test``` and i've found that to be the only appropriate file(if you had another one in mind, let me know!).
- Moved `Constraint` instead of the `if` check to reduce git diff.

https://github.com/llvm/llvm-project/pull/213660