[Sema] Expose a control flag for integer to pointer ext warning

While building openJDK11u, it seems that some of the code in the
native core libraries make liberal use of integer to pointer
comparisons. We currently have no flag to disabled this warning.
This add such a flag.

Patch by Kader (abdoul-kader keita)

Differential Revision: https://reviews.llvm.org/D56241 

llvm-svn: 351082