[clang] Handle lround/llround builtins

As for other floating-point rounding builtins that can be optimized
when build with -fno-math-errno, this patch adds support for lround
and llround.  It currently only optimize for AArch64 backend.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D61392

llvm-svn: 360896