[CodeGen] Disable UBSan for coroutine functions

Summary:
As explained in http://lists.llvm.org/pipermail/llvm-dev/2018-March/121924.html,
the LLVM coroutines transforms are not yet able to move the
instructions for UBSan null checking past coroutine suspend boundaries.
For now, disable all UBSan checks when generating code for coroutines
functions.

I also considered an approach where only '-fsanitize=null' would be disabled,
However in practice this led to other LLVM errors when writing object files:
"Cannot represent a difference across sections". For now, disable all
UBSan checks until coroutine transforms are updated to handle them.

Test Plan:
1. check-clang
2. Compile the program in https://gist.github.com/modocache/54a036c3bf9c06882fe85122e105d153
   using the '-fsanitize=null' option and confirm it does not crash
   during LLVM IR generation.

Reviewers: GorNishanov, vsk, eric_niebler, lewissbaker

Reviewed By: vsk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D44672

llvm-svn: 368675