[clang][test] Add test for incompatible cv-qualified reference types in...

[clang][test] Add test for incompatible cv-qualified reference types in conversion function template (#81950)

We currently lack test coverage for
[SemaTemplateDeduction.cpp#L1619-L1620](https://github.com/llvm/llvm-project/blob/fe20a75/clang/lib/Sema/SemaTemplateDeduction.cpp#L1619-L1620),
which handles the case where both the P type and the A type are
reference types but A is more cv-qualified than P. This is deemed
non-deduced unless both A and P are possibly cv-qualified forms of the
template parameter T. This PR adds tests for that logic.