`ObjCRuntime` and `CommentOpts.BlockCommandNames` are checked by
`ASTReader::checkLanguageOptions`, but are not part of the module
context hash. This can lead to errors when using implicit modules if
different TUs have different values for these options when using the
same module cache.
This was not hit very often due to the rare usage of
`-fblock-command-names=` and that `ObjCRuntime` is by default set by
the target triple, which is part of the existing context hash.
Michael Spencer