[Concepts] Fix overload resolution bug with constrained candidates

[Concepts] Fix overload resolution bug with constrained candidates

When doing overload resolution, we have to check that candidates' parameter types are equal before trying to find a better candidate through checking which candidate is more constrained.
This revision adds this missing check and makes us diagnose those cases as ambiguous calls when the types are not equal.

Fixes GitHub issue https://github.com/llvm/llvm-project/issues/53640

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D123182