[clang] [C++20][Modules] Fix merging of anonymous members of class templates. (PR #155948)

[Michael Park via cfe-commits]

mpark wrote:

> Now it looks much better.

@ChuanqiXu9 One more tweak here... `field_empty()` and family loads fields, which messes with some other assumptions and triggers an assertion failure. The PR now introduces a `noload_field_empty()` and family and uses that instead, which addresses the issue. I think it's also "what we mean". That is, we want to check that we have some fields currently without loading further.

I also tried just not checking the `field_empty()` condition, and relying on `numberAnonymousDeclsWithin` to be a no-op on an empty class. The problem here is that `numberAnonymousDeclsWithin` currently calls `decls()`, which triggers loading and triggers the same issue. Changing this to `noload_decls()` seems to do the trick, but it's not obvious that the usage of it in [ASTWriter.cpp](https://github.com/llvm/llvm-project/blob/e3c7b7f806559a361d2cf8374d65230c75bb5829/clang/lib/Serialization/ASTWriter.cpp#L7031) requires `decls()` semantics or not.

I'm proposing to go with the `noload_field_empty()` approach here.

https://github.com/llvm/llvm-project/pull/155948