swift234

Make sure that generated selector symbols actually get uniqued.

First, keep track of each of the selectors we emit and dump them into the
llvm.used global so that they don't get thrown away by the
optimizer. Second, emit Objective-C module-level named metadata so
that the linker knows it needs to unique selectors. Otherwise,
uniquing doesn't happen when Swift code is compiled into a separate
dylib.


Swift SVN r3287