[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

llvm-svn: 307446