The functionality was originally added in D95265: [lld-macho] Force-loading should share code path with regular archive loads, 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: [lld-macho] LTO: Unset VisibleToRegularObj where possible, which will hide irrelevant