Revisiting the pointer conversions

[benrimmington]

@John_McCall

• Allowing an implicit conversion from Array/String to a pointer is quite out of keeping with Swift's normal strong-typing rules. We've seen program bugs and memory corruption in the wild caused by unintended use of this feature.

The _convertConstStringToUTF8PointerArgument function currently creates a temporary array. If the argument is a string literal, users might expect it to behave like a static string. This should be possible with the new String ABI.

@John_McCall

• The implicit conversion from Array/String should be deprecated in favor of something that requires an explicit syntactic element (even if it's just &).

Should string literal arguments be exempted from the explicit & syntax?