[clang][codegen] Skip adding default function attributes on intrinsics.

- After loading builtin bitcode for linking, skip adding default
  function attributes on LLVM intrinsics as their attributes are
  well-defined and retrieved directly from internal definitions. Adding
  extra attributes on intrinsics results in inconsistent result when
  `-save-temps` is present. Also, that makes few optimizations
  conservative.

Differential Revision: https://reviews.llvm.org/D87761