Sema: support __declspec(dll*) on ObjC interfaces

Extend the __declspec(dll*) attribute to cover ObjC interfaces.  This was
requested by Microsoft for their ObjC support.  Cover both import and export.
This only adds the semantic analysis portion of the support, code-generation
still remains outstanding.  Add some basic initial documentation on the
attributes that were previously empty.  Tweak the previous tests to use the
relative expected-warnings to make the tests easier to read.

llvm-svn: 275610