Supporting LIBPROTOBUF_EXPORTS in clang

[m0hamed]

PROTOBUF_USE_DLLS and LIBPROTOBUF_EXPORTS are currently only defined/used with MSVC. Protobuf is used in chrome/chromium where we patch in this support downstream. However with the new addition of exporting template instantiations, this requires even more patching thus I thought we should just upstream the changes.

I created this issue to discuss the associated pull request #6535 since it was mentioned in the contributing doc to do so for complex changes (I am not sure if this applies here, but just in case). I would also suggest renaming PROTOBUF_USE_DLLS to something more reflective of the fact that it now also works on non-windows, however I did not do the change since this is something that is visible to third party users of the library rather than just internal macro definitions.

Note: the portion of this cl related to template exporting macros was not written by me but copied from the chromium repo with minor modifications. However it was written by a google engineer and is also covered by the same CLA as this project.

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