[compiler-rt] Add ubsan interface header.

This is to document __ubsan_default_options().

Reviewed By: vitalybuka

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

llvm-svn: 371822
GitOrigin-RevId: 1ae9e6918d94ec0e296c7753cd3f1a7c22cb3153
2 files changed