It seems like we currently support string literals in Cmm, so we can use
__LINE__ CPP macro in assertion macros. This improves error messages
that previously looked like

    ASSERTION FAILED: file (null), line 1302

(null) part now shows the actual file name.

Also inline some single-use string literals in PrimOps.cmm.

Reviewers: bgamari, simonmar, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4862