swift234

Teach ClangImporter to import __attribute__((unavailable)) as @availability(*,unavailable).

This is a direct translation which happens when a Clang declaration
gets translated to a Swift declaration.  This changed, coupled
with the current @availability checking (which is still limited)
now prohibits cases such as using 'NSDeallocateObject()' or
'- (BOOL) allowsWeakReference' from Swift.

Interestingly, it doesn't catch uses of -retain/-release yet, because
those methods are marked unavailable in the NSObject *protocol*.
While the attributes are being mapped over, the @availability
checking needs to be enhanced to replicate more of what Clang does
for this case.

Swift SVN r15643