Added warning for unary minus used with unsigned type

Summary:
Inspired by MSVC, which found some occurrences of this expression on our code base.

Fixes PR38950

Reviewers: rsmith, craig.topper, spatel, RKSimon, aaron.ballman, thakis

Reviewed By: rsmith

Subscribers: joerg, Quuxplusone, lebedev.ri, craig.topper, RKSimon, cfe-commits

Differential Revision: https://reviews.llvm.org/D52137

llvm-svn: 343560