Use a cache of the results of "GetFileAddress" from a symbol in the Comparator...

Use a cache of the results of "GetFileAddress" from a symbol in the Comparator we are using to sort the various lookup indices by symbol address.  When we switched to weak pointers,
this lookup got slightly slower.  Not enough to matter for most uses, but in the sort algorithm it does matter.

llvm-svn: 155873