[WebAssembly] Remove a second parameter from toString().

toString(T) is a stringize function for an object of type T. Each type
that has that function defined should know how to stringize itself, and
there should be one string representation of an object. Passing a
"supplemental" argument to toString() breaks that princple. We shouldn't
add a second parameter to that function.

Differential Revision: https://reviews.llvm.org/D44323

llvm-svn: 327182