[Diagnostics] Check integer to floating point number implicit conversions

Summary:
GCC already catches these situations so we should handle it too.

GCC warns in C++ mode only (does anybody know why?). I think it is useful in C mode too.

Reviewers: rsmith, erichkeane, aaron.ballman, efriedma, xbolva00

Reviewed By: xbolva00

Subscribers: efriedma, craig.topper, scanon, cfe-commits

Differential Revision: https://reviews.llvm.org/D52835

llvm-svn: 346865