swift234

[codesize] Keep _assertionFailed in the swift dylib.

This commit annotates _assertionFailed with an attribute
that will keep in in the swift dylib. We don't care about
the performance of this function because it happens when
the program crashes. Keeping it in the swift dylib helps
in reducing the code size of the user app because it keeps
more of the printing and string.utf8 logic in the dylib.

Swift SVN r31506