For PS4 development we support dllimport/export annotations in source code. This patch enables the dllimport/export attributes on PS4 by adding a new function to query the triple for whether dllimport/export are supported and using that function to decide whether these attributes are supported. This means we can drop the use of "TargetArch" in the .td file which is an improvement as dllimport/export support isn't really a function of the architecture.
I have included a simple codgen test to show that the attributes are accepted and have an effect on codegen. I have also enabled the DLLExportStaticLocal and DLLImportStaticLocal attributes, which we support downstream. However, I am unable to write a test for these attributes until other patches for PS4 dllimport/export handling land upstream. Whilst writing this patch I noticed that, as these attributes are internal, they do not need to be target specific (when these attributes are added internally in Clang the target specific checks have already been run); however, I think leaving them target specific is fine because it isn't harmful and they "really are" target specific even if that has no functional impact.