Don't suppress warning about dllimport on typedefs etc. in MicrosoftExt mode

It's true the MSVC doesn't warn about dllimport when applied to e.g. a typedef,
but that applies to dllexport too. I'd like us to be consistent, and I think
the right thing to do is to warn.

The original test that came with implementing the old behaviour doesn't provide
a good motivation, and it said it was checking that we're not repoting an *error*,
which is still true since this is just a warning.

There are plenty of tests e.g. in Sema/dllimport.c to check that we do warn
about dllimport on non functions or variables.

Differential Revision: http://reviews.llvm.org/D3832