SR-14256: fixes swift-frontend crash

[tschuett]

Replace assert with an error message.

Resolves SR-14256.

I would appreciate some guidance on how to create a test case.

`./swift-frontend -emit-supported-features ./swift

:0: error: not a swift file: './swift'`

[CodaFi]

I don't believe this is the correct solution - it merely silences a symptom of the real problem. I believe you want -emit-supported-features to take a different path, or perhaps report something different to FrontendOptions::shouldActionOnlyParse instead.

[tschuett]

I am wondering, why you have to pass a file at all? It is not reading the file.

It was introduced in:
#34645
by @nkcsgexi

[CodaFi]

I am wondering, why you have to pass a file at all?

EDIT: Seems like this flag is intended as a kind of supplementary output flag for a frontend compilation mode, not a flag that you can just use to dump out the version and keep moving.

[tschuett]

Well ./swift-frontend -emit-supported-features does not work. It requires an input file.

[nkcsgexi]

You could give the invocation any input file, e.g. %S

[tschuett]

But the input file is ignored?

[CodaFi]

@nkcsgexi Is this intended to be a frontend mode or a supplementary output? If it's the former, then we need to make sure that no input files come along with the invocation. If it's the latter we need to remove the frontend mode.