Remove `SymbolFileDWARF *` when there is already `DWARFUnit *`

In D61502#1503247 @clayborg suggested that SymbolFileDWARF *dwarf2Data is
really redundant in all the calls with also having DWARFUnit *cu. So remove it.

One `SymbolFileDWARF *` nullptr check
(DWARFDebugInfoEntry::GetDIENamesAndRanges) could be removed, other two nullptr
checks (DWARFDebugInfoEntry::GetName and DWARFDebugInfoEntry::AppendTypeName)
need to stay in place (now for `DWARFUnit *`).

Differential Revision: https://reviews.llvm.org/D62011

llvm-svn: 361277