[clang][Interp] Properly identify not-yet-defined functions

Since we now handle functions without a body as well, we can't just use
getHasBody() anymore. Funtions that haven't been defined yet are those
that don't have a body *and* aren't valid.

Also, just pass the information about whether the Function has a body or
not along from the FunctionDecl.

Differential Revision: https://reviews.llvm.org/D141591