[CodeGen] Add lround/llround builtins

This patch add the ISD::LROUND and ISD::LLROUND along with new
intrinsics.  The changes are straightforward as for other
floating-point rounding functions, with just some adjustments
required to handle the return value being an interger.

The idea is to optimize lround/llround generation for AArch64
in a subsequent patch.  Current semantic is just route it to libm
symbol.

llvm-svn: 360889