[Sema] List conversion validate character array.

The function `TryListConversion` didn't properly validate the following
part of the standard:

    Otherwise, if the parameter type is a character array [... ]
    and the initializer list has a single element that is an
    appropriately-typed string literal (8.5.2 [dcl.init.string]), the
    implicit conversion sequence is the identity conversion.

This caused the following call to `f()` to be ambiguous.
    void f(int(&&)[1]);
    void f(unsigned(&&)[1]);

    void g(unsigned i) {
      f({i});
    }

This issue only occurs when the initializer list had one element.

Differential Revision: https://reviews.llvm.org/D87561