not: Only consider exit code 3 to be a crash with --crash

This fixes Clang's test/Index/comment-xml-schema.c with Cygwin's
xmllint.exe, which uses exit(3) for XML validation failure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211550 91177308-0d34-0410-b5e6-96231b3b80d8