swift234

[IRGen] Mark all visible external entities as 'used'.

...so that they don't get dead-stripped out of an executable...
...so that they can be accessed by unit tests (or in-process plug-ins).

In Swift, marking something 'public' is a deliberate action (unlike in C),
so anything marked 'public' should be left in the final binary...even when
we're building an executable.

We currently /also/ mark the symbols for internal and private decls as
external as well, so they also won't be stripped. Hopefully that will
change soon.

rdar://problem/18173029

Swift SVN r21815