For MS ABI, emit dllexport friend functions defined inline in class

For MS ABI, emit dllexport friend functions defined inline in class

...as that is apparently what MSVC does.  This is an updated version of r263738,
which had to be reverted in r263740 due to test failures.  The original version
had erroneously emitted functions that are defined in class templates, too (see
the updated "Handle friend functions" code in EmitDeferredDecls,
lib/CodeGen/ModuleBuilder.cpp).  (The updated tests needed to be split out into
their own dllexport-ms-friend.cpp because of the CHECK-NOTs which would have
interfered with subsequent CHECK-DAGs in dllexport.cpp.)

Differential Revision: http://reviews.llvm.org/D18430

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@264841 91177308-0d34-0410-b5e6-96231b3b80d8