[ubsan-minimal] Make the interface more compatible with RTUBSan

This eliminates a few inconsistencies between the symbol sets exported
by RTUBSan and RTUBSan_minimal:

  * Handlers for nonnull_return were missing from the minimal RT, and
    are now added in.

  * The minimal runtime exported recoverable handlers for
    builtin_unreachable and missing_return. These are not supposed to
    exist, and are now removed.

llvm-svn: 313614