Use named temporaries for directional labels.

Use named temporaries for directional labels.

Directional labels can show up in symbol tables (and we have a llvm-mc test for
that). Given that, we need to make sure they are named.

With that out of the way, use setUseNamesOnTempLabels in llvm-mc so that it
too benefits from the memory saving.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239914 91177308-0d34-0410-b5e6-96231b3b80d8