[compiler-rt] Add ubsan interface header.

[pgousseau]

Summary

This is to document __ubsan_default_options().

compiler-rt/trunk/include/CMakeLists.txt

 if (COMPILER_RT_BUILD_SANITIZERS)
   set(SANITIZER_HEADERS
     sanitizer/allocator_interface.h
     sanitizer/asan_interface.h
     sanitizer/common_interface_defs.h
     sanitizer/coverage_interface.h
     sanitizer/dfsan_interface.h
     sanitizer/hwasan_interface.h
     sanitizer/linux_syscall_hooks.h
     sanitizer/lsan_interface.h
     sanitizer/msan_interface.h
     sanitizer/netbsd_syscall_hooks.h
     sanitizer/scudo_interface.h
     sanitizer/tsan_interface.h
     sanitizer/tsan_interface_atomic.h
+    sanitizer/ubsan_interface.h
     )
   set(FUZZER_HEADERS
     fuzzer/FuzzedDataProvider.h
     )
 endif(COMPILER_RT_BUILD_SANITIZERS)
 
 if (COMPILER_RT_BUILD_XRAY)
   set(XRAY_HEADERS

compiler-rt/trunk/include/sanitizer/ubsan_interface.h

+//===-- sanitizer/ubsan_interface.h -----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file is a part of UBSanitizer (UBSan).
+//
+// Public interface header.
+//===----------------------------------------------------------------------===//
+#ifndef SANITIZER_UBSAN_INTERFACE_H
+#define SANITIZER_UBSAN_INTERFACE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+/// User-provided default option settings.
+///
+/// You can provide your own implementation of this function to return a string
+/// containing UBSan runtime options (for example,
+/// <c>verbosity=1:halt_on_error=0</c>).
+///
+/// \returns Default options string.
+const char* __ubsan_default_options(void);
+
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
+#endif  // SANITIZER_UBSAN_INTERFACE_H