Make -Wstring-plus-int warns even if when the result is not out of bounds

Summary: Patch by Arnaud Bienner

Reviewers: sylvestre.ledru, thakis, serge-sans-paille

Reviewed By: thakis

Subscribers: arphaman, dyung, anemet, llvm-commits, cfe-commits

Differential Revision: https://reviews.llvm.org/D55382

llvm-svn: 350335