Warn on inline given aliases with functions as RHS

[odersky]

    inline given a: Conversion[String, Item] = Item(_)

will now produce this warning:

 5 |  inline given a: Conversion[String, Item] = Item(_)
   |                                             ^^^^^^^
   |An inline given alias with a function value as right-hand side can significantly increase
   |generated code size. You should either drop the `inline` or rewrite the given with an
   |explicit `apply` method.
   |----------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   | A function value on the right-hand side of an inline given alias expands to
   | an anonymous class. Each application of the inline given will then create a
   | fresh copy of that class, which can increase code size in surprising ways.
   | For that reason, functions are discouraged as right hand sides of inline given aliases.
   | You should either drop `inline` or rewrite to an explicit `apply` method. E.g.
   |
   |     inline given Conversion[A, B] = x => x.toB
   |
   | should be re-formulated as
   |
   |     inline given Conversion[A, B] with
   |       def apply(x: A) = x.toB
   |

Fixes #16497
Alternative to #16498

[smarter]

Otherwise LGTM.