Fix header namespace pollution.

[nlutsenko]

This removes using namespace ...; from headers, as such reducing header namespace pollution.
Whilst not critical for MoltenVK itself, when incorporated in a larger project makes sure that symbols/functionality is addressed appropriately and items within mvk namespace aren't considered to be part of the global namespace.

Also enabled the warning within xcodeproj, so it's visible when this is added again.

[CLAassistant]

All committers have signed the CLA.

[billhollings]

Thanks for submitting this.

LGTM.

Since you are modifying WARNING_CFLAGS, would you mind also adding the -Wunused-but-set-variable that you identified in PR #2370, please?

This will trigger a warning in the MVKSampler constructor. To fix that, can you also remove the two lines there referencing pSampReductInfo, please? Please leave the empty case statement though, to highlight that option exists and may be needed at a later time.

After your PR #2370, I was going to issue a follow up PR with those changes, but you might as well close the loop here.

[billhollings]

Since you are modifying WARNING_CFLAGS, would you mind also adding the -Wunused-but-set-variable that you identified in PR #2370, please?

This will trigger a warning in the MVKSampler constructor. To fix that, can you also remove the two lines there referencing pSampReductInfo, please? Please leave the empty case statement though, to highlight that option exists and may be needed at a later time.

I've included these additional items in PR #2382.