swift234

Use 'common' linkage for force-load symbols rather than 'weak_odr'.

As suggested by John. This results in the symbols being coalesced at static
link time into a single strong symbol in a built dylib. Fortunately we don't
care about the contents of these global symbols, so common linkage's
forced zero-initialization is fine. (Hey, C's tentative definitions are
useful for something after all.)

rdar://problem/20072051

Swift SVN r25804