[SymbolFile] SymbolFileDWARF::ParseLineTable should lock its module

As of svn rL367298, SymbolFileDWARF locks the module in many cases where
it needs to parse some aspect of the DWARF symbol file.
SymbolFileDWARF::ParseLineTable needs to lock the module because
SymbolVendor::ParseLineTable no longer locks it.

llvm-svn: 367358