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

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

llvm-svn: 209546