Fix buildbots broken by r254508

g++ 4.7 does not allow an inline defaulted virtual destructor to be overridden,
giving the error "looser throw specifier for ... overridding ~SCEVPredicate()
noexcept (true)" (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53613).

The work-around given in the bug report above has been utilised here.

llvm-svn: 254511