[compiler-rt][ubsan] Implicit Conversion Sanitizer - integer truncation - compiler-rt part

Summary:
This is a compiler-rt part.
The clang part is D48958.

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

Reviewers: #sanitizers, samsonov, vsk, rsmith, pcc, eugenis, kcc, filcab

Reviewed By: #sanitizers, vsk, filcab

Subscribers: llvm-commits, eugenis, filcab, kubamracek, dberris, #sanitizers, regehr

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D48959

llvm-svn: 338287