If multiple swift files are compiled together, then guessing as to the
file when we emit IR obviously doesn't work. Find the filename when we
generate a function's coverage map and propagate it through SIL.
Swift SVN r25436
Keeping a reference to the function here is dangerous. We only
actually care about the name, so save ourselves a copy of that
instead.
This fixes a crash that seems to happen only when the coverage data is
very large.
Swift SVN r25433
