Revert "Turn off extern templates for most uses."

Turning off explicit template instantiation leads to a pretty
significant build time and code size cost. We're better off dealing
with ABI incompatibility issues that come up in a less heavy handed
way.

This reverts commit r189610.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215740 91177308-0d34-0410-b5e6-96231b3b80d8