[lld-macho][nfc] Test that -ObjC will import bitcode with category sections

The functionality was originally added in {D95265}, but the test in that
diff only checked if `-ObjC` would cause bitcode containing ObjC class
symbols to be loaded. It neglected to test for bitcode containing
categories but no class symbols.

This diff also changes the lto-archive.ll test to use `-why_load`
instead of inspecting the output binary's symbol table. This is
motivated by the stacked diff {D99105}, which will hide irrelevant
bitcode symbols.

Reviewed By: #lld-macho, oontvoo

Differential Revision: https://reviews.llvm.org/D99215