If a `[[no_unique_address]]` field has zero size according to Clang, and field has a type that isn't representable in Swift, Swift would previously try to add an opaque field of size 1 for it.
This is wrong and was causing crashes for `std::string` while emitting a padding field:
```
_LIBCPP_NO_UNIQUE_ADDRESS ::std::__compressed_pair_padding<T1> _LIBCPP_CONCAT3(__padding1_, __LINE__, _);
```
rdar://151941799
In #80786, we started importing certain padded fields as opaque blobs.
Part of this logic involved querying those fields' ASTRecordLayout.
However, dependent types (which are imported symbolically) do not have
an ASTRecordLayout, so calling Clang's getASTRecordLayout() would lead
to an assertion error for class templates where a no_unique_address
field is some kind of dependent C++ record type.
This patch avoids the field padding check during symbolic import mode
because that check is only relevant for codegen anyway.
rdar://150067288
Swift does not support storing fields in the padding of the previous
fields just yet, so let's not import fields like that from C++.
Represent them as opaque blobs instead.
Fixes#80764
rdar://149072458
Zero sized fields are messing up the offset calculations when we import
C++ fields to Swift. We assume that the size of the field is determined
by the type of the field. This is not true for fields marked with
no_unique_address. Those fields can have 0 size while the
sizeof(decltype(field)) is still 1.
rdar://143907490
