Add "extern template" declarations now that we use explicit instantiation.

Add "extern template" declarations now that we use explicit instantiation.

This is supported by gcc and clang, but guarded by a macro for MSVC 2008.

The extern template declaration is not necessary but generally good
form. It can avoid extra instantiations of the template methods
defined inline.

The EXTERN_TEMPLATE_INSTANTIATION macro could probably be generalized to
handle multiple template parameters if someone thinks it's worthwhile.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158840 91177308-0d34-0410-b5e6-96231b3b80d8