[dotest] Use in-tree dsymutil on Darwin

Summary:
With the upstream implementation of dsymutil containing almost all
functionality from the one shipped with Xcode, we want to use the
in-tree version for running the test suite.

This will also allow us to re-enable TestUnicodeSymbols which was
failing because of the discrepancy in how Unicode symbols were hashed in
lldb and older versions of dsymutil.

Reviewers: aprantl, davide, jingham, labath

Subscribers: mgorny, llvm-commits, lldb-commits

Differential Revision: https://reviews.llvm.org/D45518

llvm-svn: 329889