clang-cl: Treat dllimport explicit template instantiation definitions as...

clang-cl: Treat dllimport explicit template instantiation definitions as declarations (PR27810, PR27811)

This matches what MSVC does, and should make compiles faster by avoiding to
unnecessarily emit a lot of code.

Differential Revision: http://reviews.llvm.org/D20608

llvm-svn: 270748