[ODRHash] static_cast and Stmt hashing.

Add support for static_cast in classes.  Add pointer-independent profiling for
Stmt's, sharing most of the logic with Stmt::Profile.  This is the first of the
deep sub-Decl diffing for error messages.

Differential Revision: https://reviews.llvm.org/D21675

llvm-svn: 295890
6 files changed