build: hide symbols using CMake

[compnerd]

This uses the CMake settings to correctly select the visibility
attribute for the runtime.

Addresses rdar://63454568

[compnerd]

@edymtt - I think that this is a better approach to what you were trying to do. I'd still like to understand how this is a performance thing. These are not weak symbols, they are COMDATed symbols. However both of those do not incur a penalty - the weak symbol just means that if one is not provided it will be used, but the resolution is done once at load time.

[compnerd]

@swift-ci please test

[compnerd]

CC: @drexin

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 941681f

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 941681f

[mikeash]

@compnerd This came from an internal verifier which says that weak symbols can impact app launch time. I'm not completely clear as to why, but I guess dyld has to do more work.

[compnerd]

@mikeash Yes, that’s absolutely correct, the original statement of the reason just made it seem like the change caused a regression at runtime. Eric helped clear that up. Just a miscommunication on the actual results of the change.

[mikeash]

@compnerd Yeah, it's always a bit tricky and weird when internal stuff bubbles up like this. Glad we're all on the same page now.

[edymtt]

@swift-ci please test

[compnerd]

@edymtt Ill go ahead and merge this, and then follow up with the tests for ensuring that this doesn't happen again in the future. Thanks for the help!