Make up a module name for c-- files

Summary:
We used to pass a bottoming Module to the NCG, which resulted in panics
when -v was used due to debug output (see Trac #11784). Instead we make up
a module name. This is a bit scary since PIC.howToAccessLabel might
actually use the Module, but if it wasn't crashing before I suppose it's
fine.

Test Plan: touch hi.cmm; ghc -v2 -c -dcmm-lint hi.cmm

Reviewers: austin, simonmar

Reviewed By: simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2864

GHC Trac Issues: Trac #11784