[lldb] Ensure that dwo/dwp are not double-indexed

DWO/DWP should not be indexed directly.
Instead, the corresponding base file should be used.
This diff adds an assert to DWARFCompileUnit::Index
and adjusts the methods 
SymbolFileDWARF::FindCompleteObjCDefinitionTypeForDIE,
SymbolFileDWARF::GetObjCMethodDIEOffsets accordingly.

Differential revision: https://reviews.llvm.org/D39825

llvm-svn: 318554