DWARF: Simplify SymbolFileDWARF::GetDWARFCompileUnit

Summary:
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

llvm-svn: 362783