Untangle a snarl that I discovered when updating the mangler,

starting in getCurrentFunctionEHName.  Among other problems,
we would try to privative a "foo.eh" label, but end up emitting
the label as _Lfoo.eh instead of L_foo.eh on darwin.  This is really
bad, and the linker has always tolerated these labels existing.
For now, just emit them as _foo.eh.

This patch also fixes problems with ".eh" labels on unnamed
functions and eliminates two strangely defined TargetAsmInfo
hooks.

llvm-svn: 76231