[clang] Turn -fno-builtin flag into an IR Attribute

Summary:
This is a follow up on https://reviews.llvm.org/D61634#1742154 to turn the clang driver -fno-builtin flag into an IR attribute.
I also investigated pushing the attribute earlier on (in Sema) but it looks like this patch is simple and will cover all function calls.

Reviewers: aaron.ballman, courbet

Subscribers: cfe-commits, tejohnson

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71193