Added a special test for getObjCClassByMangledName; this needs testing
separately as it uses the DecodedMetadataBuilder, which doesn't get exercised
by the normal demangling tests.
Added all the test cases from rdar://63485806, rdar://63488139, rdar://63496478,
rdar://63410196 and rdar://68449341. The test cases from rdar://63485806 are
disabled for now because the problem there is the error handling mechanism (or
lack thereof), rather than us not handling errors.
Fixes the remaining cases from
rdar://63488139
rdar://63496478