[clang] Handle lrint/llrint builtins

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

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D62019

llvm-svn: 361878