Use printable names to implement directional labels.

This changes the implementation of local directional labels to use a dedicated
map. With that it can then just use CreateTempSymbol, which is what the rest
of MC uses.

CreateTempSymbol doesn't do a great job at making sure the names are unique
(or being efficient when the names are not needed), but that should probably
be fixed in a followup patch.

This fixes pr18928.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203826 91177308-0d34-0410-b5e6-96231b3b80d8