[dsymutil] Reapply r245960.

[dsymutil] Reapply r245960.

There was an issue in the test setup because the test requires an arch that
wasn't filtered by the lit.local.cfg, but given the set of bots that failed,
I'm not confident this is the (only) issue. So this commit also adds more
output to the test to help me track down the failure if it happens again.

Original commit message:
[dsymutil] Rewrite thumb triple names in user visible messages.

We autodetect triples from the input file(s) while reading the mach-o debug map.
As we need to create a Target from those triples, we always chose the thumb
variant (because the arm variant might not be 'instantiable' eg armv7m). The
user visible architecture names should still be 'arm' and not 'thumb' variants
though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245988 91177308-0d34-0410-b5e6-96231b3b80d8