[clang-tidy] Optimize query in bugprone-exception-escape

Checking whether a functions throws indirectly may be very expensive because it
needs to visit its whole call graph. Therefore we should first check whether the
function is forbidden to throw and only check whether it throws afterward. This
also seems to solve bug https://bugs.llvm.org/show_bug.cgi?id=39167 where the
execution time is so long that it seems to hang.

Differential Revision: https://reviews.llvm.org/D53187

llvm-svn: 344444