Add missing roundtointegral builtin functions for some FP instructions to be...

Add missing roundtointegral builtin functions for some FP instructions to be generated from C-written codes

To generate FROUND instructions in https://reviews.llvm.org/D143982, we need to use llvm intrinsics in IR files. Now I add some corresponding builtin functions to make sure these roundtointegral instructions can be generated from C codes.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D144935