[Bug ld/28973] New: Improve error message for missing vtable

[eyalroz1 at gmx dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28973

            Bug ID: 28973
           Summary: Improve error message for missing vtable
           Product: binutils
           Version: 2.35.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: eyalroz1 at gmx dot com
  Target Milestone: ---

Related GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42540

If we write the following program:
```
struct A {
    virtual void foo() { }
};

struct B : A {
    void foo() override;
};

int main() {
    B b;
}
```
it compiles, but fails to link:
```
<source>:10: undefined reference to `vtable for B'
```
(see https://godbolt.org/z/Mzx3c7354)

Now, the reason the vtable is missing is that the C++ ABI (for x86_64?)
dictates that the vtable be defined in the object file which defines the "key
function" - the first non-pure, non-inline virtual member function of the
class. In this case, it's `B::foo()`.

The error message GNU ld emits is not useful enough. It should let the user
know _why_ the vtable is undefined - as the user her/himself is not supposed to
have provided a vtable; it's something else that's the problem with what they
wrote. So the message should be something like:

"undefined reference to the virtual method table for class `B`. This is due to
no object having defined the first virtual method in class `B` which isn't
inline nor pure-virtual".

or if somehow the linker can figure it out:

"undefined reference to the virtual method table for class `B`, due to
`B::foo()` being undefined"

(Obviously, the suggested wording is just a suggestion, the point is to impart
the relevant information.)

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