[ODRHash] static_cast and Stmt hashing.

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@295890 91177308-0d34-0410-b5e6-96231b3b80d8