[compiler-rt][ubsan] Implicit Conversion Sanitizer - integer sign change - compiler-rt part

Summary:
This is a compiler-rt part.
The clang part is D50250.

See [[ https://bugs.llvm.org/show_bug.cgi?id=21530 | PR21530 ]], https://github.com/google/sanitizers/issues/940.

Reviewers: vsk, filcab, #sanitizers

Reviewed By: filcab, #sanitizers

Subscribers: mclow.lists, srhines, kubamracek, dberris, rjmccall, rsmith, llvm-commits, regehr

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D50251

llvm-svn: 345659