[lldb][NFC] Simplify ClangExternalASTSourceCallbacks

This class is only used by the ClangASTContext so we might as well
simplify this whole logic by just passing a ClangASTContext instead
of a list of callbacks and a void* pointer. If we ever need this
to support other classes then we can define some interface that
ClangASTContext implements but for now this isn't needed.

I also removed any code for m_callback_find_by_name as this was
always a nullptr in LLDB and removed all overriden implementations
that just redefined the default no-op implementation that the
ExternalASTSource provides.

Also removed the assert.h workarounds.