[+0-args][runtime] Add temporary runtime swift convention macros.

[gottesmm]

These are temporary staging macros to ease having a runtime that supports both
+0 and +1 conventions for functions exposed as Swift level functions in the
stdlib (and thus needing to follow the swift convention). The macros values are
toggled by the argument SWIFT_ENABLE_GUARANTEED_NORMAL_ARGUMENTS and thus have
values described via the following table:

| SWIFT_ENABLE_GUARANTEED_NORMAL_ARGUMENT | FALSE                         | TRUE                          |
|-----------------------------------------+-------------------------------+-------------------------------|
| SWIFT_NS_RELEASES_ARGUMENT              | NS_RELEASES_ARGUMENT          | ""                            |
| SWIFT_CC_PLUSONE_GUARD(...)             | do { __VA_ARGS__ ; } while(0) | ""                            |
| SWIFT_CC_PLUSZERO_GUARD(...)            | ""                            | do { __VA_ARGS__ ; } while(0) |

Thus instead of having to write an ugly #ifdef multiple times in each function
(for the arguments, destroys, and retains), we can just use these macros
instead.

In a subsequent commit I am going to cleanup the changes I made in the runtime
already to use these macros. So this is a NFC change.

rdar://34222540

[gottesmm]

FYI, I already ran this by @gparker42.

[gottesmm]

@swift-ci smoke test and merge