builtin_constant_p support in LLVM IR

I found this bug report on builtin_constant_p support in LLVM.

http://llvm.org/bugs/show_bug.cgi?id=4898

It looks like there was some interest in this, and that there was even a patch doing some work on that, but
it seems to have been abandoned. I couldn't tell if this was really because the community decided that support
for builtin_constant_p in LLVM IR was undesirable, or if the person working on this just lost interest.

Kevin Smith