[lldb] Remove clang classes from lldb-forward.h

Summary:
lldb-forward.h is convenient in many ways, but having clang-based
class forward declarations in there makes it easy to proliferate uses of clang
outside of plugins. Removing them makes you much more conscious of when
you're using something from clang and marks where we're using things
from clang in non-plugins.

Differential Revision: https://reviews.llvm.org/D73935