[compiler-rt] Add ubsan interface header.

This is to document __ubsan_default_options().

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D67503

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@371822 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed