[analyzer] Add NumberObjectConversion checker.

When dealing with objects that represent numbers, such as Objective-C NSNumber,
the language provides little protection from accidentally interpreting
the value of a pointer to such object as the value of the number represented
by the object. Results of such mis-interpretation may be unexpected.

The checker attempts to fill this gap in cases when the code is obviously
incorrect.

With "Pedantic" option enabled, this checker enforces a coding style to
completely prevent errors of this kind (off by default).

Differential Revision: https://reviews.llvm.org/D22968

llvm-svn: 284473