Add InlinesVisibility API

[tdesveauxPKFX]

As said in #1044, inlines visibility should have it's own API.

[TurkeyMan]

Why does this need its own API? Why can't this be expressed with visibility?
It feels very unlikely that you would ever supply -fvisibility-inlines-hidden without also supplying -fvisibility=hidden.

[tdesveauxPKFX]

We looked into it when I submitted #1044 and it seems -fvisibility=hidden "includes" the behavior of -fvisibility-inlines-hidden as far as I know. If you know more about it, I'd be interested.

Otherwise, I opted for a new API as you may want to set -fvisibility-inlines-hidden and let's say -fvisibility=internal.

From gcc documentation

Lastly, there's one other new command line switch: -fvisibility-inlines-hidden. This causes all inlined class member functions to have hidden visibility, causing significant export symbol table size & binary size reductions though not as much as using -fvisibility=hidden. However, -fvisibility-inlines-hidden can be used with no source alterations, unless you need to override it for inlines where address identity is important either for the function itself or any function local static data.

Again, this is not an option I'm that much knowledgeable about so if you have further information about it, I am open to improve this.

[tvandijck]

@TurkeyMan do you still have objections? otherwise go ahead and merge this one, LGTM.