Add EXTWARN Diagnostic class.

 - Like EXTENSION but always generates a warning (even without
   -pedantic).
 - Updated ptr -> int, int -> ptr, and incompatible cast warnings to
   be EXTWARN.
 - Other EXTENSION level diagnostics should be audited for upgrade.
 - Updated several test cases to fix code which produced unanticipated
   warnings.

llvm-svn: 54335