Try a completely different approach to this type trait to appease older

Try a completely different approach to this type trait to appease older
compilers. It seems that GCC 4.3 (and likely older) simply aren't going
to do SFINAE on non-type template parameters the way Clang and modern
GCCs do...

Now we detect the implicit conversion to an integer type, and then
blacklist classes, pointers, and floating point types. This seems to
work well enough, and I'm hopeful will return the bots to life.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152227 91177308-0d34-0410-b5e6-96231b3b80d8