[LCG] Make the name of an SCC include more of the functions in it.

This makes it much easier to debug issues when the logging contains the
name of the SCC. It requires to create a temporary string, but for
logging and debugging uses that seems fine. I've added logic to try to
output all the function names with an elipsis if there are too many.
This was helpful fro me in debugging issues with the new pass manager.

llvm-svn: 273625