[clang][ubsan] Split Implicit Integer Truncation Sanitizer into unsigned and signed...

[clang][ubsan] Split Implicit Integer Truncation Sanitizer into unsigned and signed checks

Summary:
As per IRC disscussion, it seems we really want to have more fine-grained `-fsanitize=implicit-integer-truncation`:
* A check when both of the types are unsigned.
* Another check for the other cases (either one of the types is signed, or both of the types is signed).

This is clang part.
Compiler-rt part is D50902.

Reviewers: rsmith, vsk, Sanitizers

Reviewed by: rsmith

Differential Revision: https://reviews.llvm.org/D50901

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@344230 91177308-0d34-0410-b5e6-96231b3b80d8