Fix compilation warning about unused variable [NFC]

Without the fix at least clang 3.6 complains with

../tools/clang/lib/AST/ExprConstant.cpp:90:24: error: unused variable 'TI' [-Werror,-Wunused-variable]
    if (TypeInfoLValue TI = B.dyn_cast<TypeInfoLValue>())
                       ^
1 error generated.

llvm-svn: 361145