[clang] [C23] Fix treating unnamed records nested in different types as compatible. (PR #162933)

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

It is a follow-up to https://github.com/llvm/llvm-project/pull/141783

GCC behavior can be checked at https://godbolt.org/z/dPq9aTq51 I think according to N3037 unnamed records are still incompatible in C23 unless the opposite is specified. And structs like `InnerUnnamedStruct_...` aren't mentioned in the standard specifically.

A few patches are a preliminary clean-up for simple review. Planned to squash them with the actual code change because they are so small. But can do a separate PR for them.

https://github.com/llvm/llvm-project/pull/162933