Revert "unordered_map: Reuse insert logic in emplace when possible, NFC"

This reverts commit r258575.  EricWF sent me an email (no link since it
was off-list) requesting to review this pre-commit instead of
post-commit.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@258625 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed