Disable tests of -fsanitize on OpenBSD

This causes failures on buildbots:
/export/users/atombot/llvm/clang-atom-d525-fedora-rel/llvm/tools/clang/test/Driver/fsanitize.c:18:29: error: expected string not found in input
// CHECK-UNDEFINED-OPENBSD: "-fsanitize={{((signed-integer-overflow|integer-divide-by-zero|float-divide-by-zero|shift-base|shift-exponent|unreachable|return|vla-bound|alignment|null|pointer-overflow|float-cast-overflow|array-bounds|enum|bool|builtin|returns-nonnull-attribute|nonnull-attribute),?){18}"}}
                            ^
<stdin>:1:1: note: scanning from here
clang version 7.0.0 (trunk 326648)
^

This will be investigated.

llvm-svn: 326652