Rewrite DSAA::getModRefInfo to compute the mapping between caller and callee

to determine mod/ref behavior, instead of creating a *copy* of the caller
graph and inlining the callee graph into the copy.

This speeds up aa-eval on Ptrdist/yacr2 from 109.13s to 3.98s, and gives
identical results.  The speedup is similar on other programs.

llvm-svn: 20669