[objcmt] Have the modern objc migration warning change from:

[NSNumber numberWithDouble:cppb];
warning: converting to boxing syntax requires a cast

to something like:

[NSNumber numberWithDouble:cppb];
warning: converting to boxing syntax requires casting 'bool' to 'double'

This is way better to fully understand the warning.
rdar://11705106

llvm-svn: 158783