[MS] Don't statically initialize dllimport member function pointers

Summary:
r306137 made dllimport pointers to member functions non-constant. This
is correct because a load must be executed to resolve any dllimported
data. However, r306137 did not account for the use of dllimport member
function pointers used as template arguments.

This change re-lands r306137 with a template instantiation fix.

This fixes PR33570.

Reviewers: rnk, majnemer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34714

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307446 91177308-0d34-0410-b5e6-96231b3b80d8