PIC and mcmodel=large on x86 doesn't use any relocations

To follow up on this... I had a few conversations at the developer's meeting
about this. I can see that "large" is really a big hammer for what I want to
do. I'm going to relook to see if "medium" is a better fit although I think
I might want a "medium+" setting since each segment will fit into 2GB (ie
code, GOT, PLT, etc) but the initial distance between my GOT and code might
be larger and 2GB (this is the part I need to double check on what we
currently do on OpenVMS Itanium).

As for the current behavior of asking for "-fPIC -mcmodel=large" and getting
them ignored without any warning, I find that unfriendly.

Where would be the best place to filter that combination and issue some
warning? And remember, such settings might be coming from a non-Clang
frontend... I'm looking for a good starter bug for myself to dip my toes
into a real commit and this looks like a place to start.