Enhance attribute 'nonnull' to be applicable to parameters directly (infix).
This allows the following syntax:
void baz(__attribute__((nonnull)) const char *str);
void baz(const char *str) __attribute__((nonnull(1)));
This also extends to Objective-C methods.
The checking logic in Sema is not as clean as I would like. Effectively
now we need to check both the FunctionDecl/ObjCMethodDecl and the parameters,
so the point of truth is spread in two places, but the logic isn't that