DebugInfo: Refactor emitDebugPubNames/Types into a common implementation

I could fold the callers into their one call site, but the indirection
(given how verbose choosing the section is) seemed helpful.

The use of a member function pointer's a bit "tricky", but seems limited
enough, the call sites are simple/clean/clear, and there's only one use.

llvm-svn: 203619