unordered_map: Reuse insert logic in emplace when possible, NFC

An upcoming commit will add an optimization to insert() that avoids
unnecessary mallocs when we can safely extract the key type.  This
commit shares code between emplace() and insert():
- if emplace() is given a single argument, and
- value_type is constructible from that argument
so that we have a single code path for the two.

I also updated the debug version of emplace_hint() to defer to
emplace(), like the non-debug version does.

In both cases, there should be NFC here.

llvm-svn: 258575