DWARF: Simplify SymbolFileDWARF::GetDWARFCompileUnit
The DWARFCompileUnit is set as the "user data" of the lldb compile unit
directly in the constructor (see ParseCompileUnit).
This means that instead of going through unit indexes, we can just fetch
the DWARF unit directly from there.
Reviewers: clayborg, JDevlieghere
Subscribers: aprantl, jdoerfert, lldb-commits
Differential Revision: https://reviews.llvm.org/D62943