String's hashValue function is implemented in terms of Foundation's hash
function in a runtime function on darwin platforms. For non-ASCII strings we
will call str.decomposedStringWithCanonicalMapping inside this runtime function
which will allocate a new NSString and return the result in the current
autorelease pool. We implemented this function in a file compiled without ARC.
This meant that we would leak said NSString into the current active autorelease
pool.
This patch moves the implementation to a file compiled with ARC. ARC will insert
objc_retainAutoreleasedReturnValue call and on platforms that require it an
marker for the hand-off of the autoreleased return value optimization.
SR-4889
rdar://32199117
