Deprecate and remove interface keyword

[llvm-beanz]

Is your feature request related to a problem? Please describe.
HLSL interface declarations are very limited and a bit broken in DXC. Some examples are:

Given these declarations (which compile fine in isolation):

interface Animal {
    void Walk();
};

class Dog : Animal {
  int4 Legs;

  void Walk() {}
};

class Worm : Animal {
  void crawl() {}
};

Compiler Explorer

The following errors can occur with subsequent expressions.

Function parameters cannot be declared of interface type. This is true for all parameter modifier specifications (in/inout/out).
Code:

void move(Animal A) {}

Diagnostics:

error: parameter type 'Animal' is an abstract class
void move(Animal A) {}
                 ^
<source>:3:10: note: unimplemented pure virtual method 'Walk' in 'Animal'
    void Walk();
         ^

Code:

Animal A;

Diagnostics:

<source>:20:10: error: variable type 'Animal' is an abstract class
  Animal A;
         ^

This is the one case that seems to do somewhat what you would expect, except it only occurs on a declaration not at any other point.

Code:

Worm B;

Diagnostics

<source>:21:8: error: variable type 'Worm' is an abstract class
  Worm B;
       ^
<source>:3:10: note: unimplemented pure virtual method 'Walk' in 'Worm'
    void Walk();
         ^

Describe the solution you'd like
Interfaces are a bit of legacy that don't really work in HLSL. The functionality of interfaces can be provided with C++ templates using SFINAE or C++20 concepts. As we align the language in that direction we should remove the interface keyword in HLSL 202x.