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.

llvm-svn: 203826