Drop the hidden visibility from DebugHandlerBase for now.

If a class has hidden visibility all derived classes and all classes
that have it as a member must have hidden visibility too. That may
be fixable here but requires changes to quite a lot of debug info
classes.

This is also one of the things that GCC enforces aggressively while
clang ignores it, making testing more annoying than necessary.

llvm-svn: 260529