[analyzer] Add missing pre-post-statement callbacks for OffsetOfExpr.

This expression may or may not be evaluated in compile time, so tracking the
result symbol is of potential interest. However, run-time offsetof is not yet
supported by the analyzer, so for now this callback is only there to assist
future implementation.

Patch by Henry Wong!

Differential Revision: https://reviews.llvm.org/D42300

llvm-svn: 324790