Warn unsupported attributes

[Happypig375]

No description provided.

[dsyme]

Need to fix up the test and get it green, but this is great, thank you.

[KevinRansom]

@dsyme, I'm wondering if this should be an error, because if we should ever choose to implement the functionality declared by these attributes then it would be a breaking change.

[auduchinok]

@KevinRansom Making it an error where it'd previously compile fine would be a breaking change itself.

[vzarytovskii]

Just a note - a less verbose withDiagnostics can be used here, for example, like in the ErrorMessages suite:

fsharp/tests/FSharp.Compiler.ComponentTests/ErrorMessages/ClassesTests.fs

Lines 68 to 71 in dbf9a62

	|> withDiagnostics [
	(Error 767, Line 8, Col 16, Line 8, Col 23, "The type Foo contains the member 'MyX' but it is not a virtual or abstract method that is available to override or implement.")
	(Error 17, Line 8, Col 18, Line 8, Col 21, "The member 'MyX : unit -> int' does not have the correct type to override any given virtual method")
	(Error 783, Line 6, Col 11, Line 6, Col 14, "At least one override did not correctly implement its corresponding abstract member")]

[Happypig375]

@vzarytovskii How can I generate them? For withResults I can just copy from the test output.

[vzarytovskii]

@vzarytovskii How can I generate them? For withResults I can just copy from the test output.

I'm afraid, you'll have to write them manually :(
Let me see if I can serialise them into something which you can just copy-paste into withDiagnostics.