Revert "DWARF: Simplify SymbolFileDWARF::GetDWARFCompileUnit"

This reverts commit 58afc1bdebf9fa8b178d6c9d89af94c5cc091760.
This commit caused the test suite on macOS to fail many tests. It
appears that setting breakpoints is the issue. One example that fails
is the lit test Breakpoint/case-sensitive.test.

llvm-svn: 362862