Revert "Reland [clang][ASTImport] Add support for import of empty records" (#100903)

This reverts commit 88e5206f2c96a34e23a4d63f0a38afb2db044f0a. The
original change went in a while ago (last year) in
https://reviews.llvm.org/D145057. The specific reason I'm proposing a
revert is that this is now causing exactly the issue that @balazske
predicted in https://reviews.llvm.org/D145057#4164717:
> Problematic case is if the attribute has pointer to a Decl or Type
that is imported here in a state when the field is already created but
not initialized. Another problem is that attributes are added a second
time in Import(Decl *)

This now came up in the testing of LLDB support for
https://github.com/llvm/llvm-project/issues/93069. There,
`__compressed_pair`s are now replaced with fields that have an
`alignof(...)` and `[[no_unique_address]]` attribute. In the specific
failing case, we're importing following `std::list` method:
```
size_type& __sz() _NOEXCEPT { return __size_; }
```
During this process, we create a new `__size_` `FieldDecl` (but don't
initialize it yet). Then we go down the `ImportAttrs` codepath added in
D145057. This imports the `alignof` expression which then references the
uninitialized `__size_` and we trigger an assertion.

Important to note, this codepath was added specifically to support
`[[no_unique_address]]` in LLDB, and was supposed to land with
https://reviews.llvm.org/D143347. But the LLDB side of that never
landed, and the way we plan to support `[[no_unique_address]]` doesn't
require things like the `markEmpty` method added here. So really, this
is a dead codepath, which as pointed out in the original review isn't
fully sound.

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