Correct the diagnostic behavior for unreachable _Generic associations in C++

New diagnostics were added for unreachable generic selection expression
associations in ca75ac5f, but it did
not account for a difference in behavior between C and C++ regarding
lvalue to rvalue conversions. So we would issue diagnostics about a
selection being unreachable and then reach it. This corrects the
diagnostic behavior in that case.

Differential Revision: https://reviews.llvm.org/D125882