Remove useless reinterpret_casts from Stmt.cpp

Summary:
I have no idea why these were there in the first place, but now they are
certainly not necessary.

Reviewers: rsmith

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1581

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189813 91177308-0d34-0410-b5e6-96231b3b80d8