Make Argument::getArgNo() constant time, not O(#args)

getArgNo is actually hot in LLVM, because its how we check for
attributes on arguments:
  bool Argument::hasNonNullAttr() const {
    if (!getType()->isPointerTy()) return false;
    if (getParent()->getAttributes().
          hasAttribute(getArgNo()+1, Attribute::NonNull))
      return true;

It actually shows up as the 23rd hottest leaf function in a 13s sample
of LTO of llc.

This grows Argument by four bytes, but I have another pending patch to
shrink it by removing its ilist_node base.

Reviewed By: chandlerc

Subscribers: inglorion, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D31057

llvm-svn: 298003