Fix a false positive in misplaced-widening-cast

Summary:
bugprone-misplaced-widening-cast check
used to give a false warning to the
following example.

 enum DaysEnum{
    MON = 0,
    TUE = 1
    };

 day = (DaysEnum)(day + 1);
 //warning: either cast from 'int' to 'DaysEnum' is ineffective...

But i think int to enum cast is not widening neither ineffective.

Patch by dkrupp.

Reviewers: JonasToth, alexfh

Reviewed By: alexfh

Subscribers: rnkovacs, Szelethus, gamesh411, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D55255

llvm-svn: 348341