New loplugin:inlinevisible to flag inline functions marked DLLEXPORT

...which does not make sense.  On Linux and Mac OS X, they potentially end up
exported from multiple libs (weakly, though), while on Windows the potentially
even end up not emitted at all, which could cause link errors.

Change-Id: I092c9ba39e686c17b6e91581cdd4753f1c4d582f