Patch by @wecing (Chenguang Wang).

The current getFoldedSizeOf() implementation uses naive recursion, which
could be really slow when the input structure type is too complex.

This issue was first brought up in
http://llvm.org/bugs/show_bug.cgi?id=8281; this change fixes it by
adding memoization.

Differential Revision: https://reviews.llvm.org/D6594