Warn when declaring an @inline method that cannot always be inlined

[retronym]

Add an opt-in warning (under some specific -Yopt-warnings:... flag) when declaring an @inline method that cannot always be inlined.

• If the method contains exception handlers (it can only be inlined on top of an empty callsite stack)
• If the method is not final (it can only be inlined if the receiver is known precisely, or known to be a final subclass)

[lrytz]

Example in the standard library: List.dropWhile cannot be inlined b/c it invokes a private method (the nested method loop is private in bytecode): https://github.com/scala/scala/blob/v2.12.0-M3/src/library/scala/collection/immutable/List.scala#L354-L361