[Clang][CodeGen] Fix __builtin_counted_by_ref for nested struct FAMs (#182575) (#182590)

GetCountedByFieldExprGEP() used getOuterLexicalRecordContext() to find
the RecordDecl containing the counted_by count field. This walks up
through all lexically enclosing records to find the outermost one, which
is wrong when a struct with a counted_by FAM is defined nested inside
another named struct.

For example, when struct inner (containing the FAM) is defined inside
struct outer, getOuterLexicalRecordContext() resolves to struct outer
instead of struct inner. The StructAccessBase visitor then fails to
match the base expression type (struct inner *) against the expected
record (struct outer), returning nullptr. This nullptr propagates back
as the GEP result, and the subsequent dereference in
*__builtin_counted_by_ref() triggers an assertion failure in
Address::getBasePointer().

Replace getOuterLexicalRecordContext() with a walk that only traverses
anonymous structs and unions, which are transparent in C and must be
walked past. Named nested structs are independently-addressable types,
so the walk stops at them.

Add a regression test for a FAM struct defined nested inside another
struct.

This also fixes __builtin_dynamic_object_size() for FAMs in nested
structs, which was silently returning -1 (unknown) instead of computing
the correct size. Update the attr-counted-by-pr88931.c test to reflect
the now-correct dynamic object size calculation.

Fixes #182575

Signed-off-by: Kees Cook <kees@kernel.org>