[clang] Improve bit-field in ref NTTP diagnostic (#71077)

Prior to this, attempts to bind a bit-field to an NTTP of reference type
produced an error because references to subobjects in NTTPs are
disallowed. But C++20 allows references to subobjects in NTTPs generally
(see
[P1907R1](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1907r1.html)).
Without this change, implementing P1907R1 would cause a bug allowing
bit-fields to be bound to reference template arguments.

Extracted from https://reviews.llvm.org/D140996