Sema: Don't diagnose string + int if the int is value dependent

Don't send a value dependent expression into the expression evaluator,
HandleSizeof would crash.  Making HandleSizeof handle dependent types
would noisily warn about the operation even if everything turns out OK
after instantiation.

This fixes PR21848.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224240 91177308-0d34-0410-b5e6-96231b3b80d8