C++ Interop: support mutating attribute for C++ methods

[egorzhdan]

This change teaches ClangImporter to import C++ methods marked with __attribute__((__swift_attr__("mutating"))) as mutating in Swift. This is useful, for example, when a method mutates this despite being const in C++ (e.g. via const_cast).

This is a follow-up after #38618.

[egorzhdan]

@swift-ci please smoke test

[zoecarver]

Maybe a test where there are multiple swift_attr attributes. Also, you can spell it like this: struct __attribute__((swift_attr("mutating"))) X { };.

[egorzhdan]

Maybe a test where there are multiple swift_attr attributes

Good idea, I added a test.

Also, you can spell it like this: struct __attribute__((swift_attr("mutating"))) X { };.

Hmm, that currently produces mutating struct X which doesn't look right, but it also worked this way before this change :)
Do you think mutating on a struct should apply it to all members methods?

[zoecarver]

Do you think mutating on a struct should apply it to all members methods?

Hmm, I don't know. But the test looks good how it is now. I think that's the correct way to write it.

[zoecarver]

This is a great improvement. As always, thanks for all your work on interop, Egor!

Feel free to merge this whenever you feel it's ready.

[egorzhdan]

@swift-ci please smoke test

[egorzhdan]

@swift-ci please smoke test

[zoecarver]

Still looks good. 🚢 it

[egorzhdan]

@swift-ci please smoke test