[GMR] Switch the function info we store for every function to be a much

more dense datastructure. We actually only have 3 bits of information
and an often-null pointer here. This fits very nicely into a
pointer-size value in the DenseMap from Function -> Info. Then we take
one more pointer hop to get to a secondary DenseMap from GlobalValue ->
ModRefInfo when we actually have precise info for particular globals.

This is more code than I would really like to do this packing, but it
ended up reasonably cleanly laid out. It should ensure we don't hit
scaling limitations with more widespread use of GMR.

llvm-svn: 242991