[CSDiag] Remove nested type and use TC.isConvertible instead

[AnthonyLatsis]

cc @xedin

Noticed we have existing functionality for convertibility checking

[slavapestov]

@swift-ci Please smoke test

[xedin]

Looks reasonable, but involving matchTypes might not always produces the same results as simplified code TypeConvertibleChecker had.

[AnthonyLatsis]

I’ll add a leading isEqual then. Might be even better to prepend it in TC.isConvertible as a fast path if it isn’t there.

…

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On 11 Jun 2018, at 21:19, Pavel Yaskevich ***@***.***> wrote: @xedin approved this pull request. Looks reasonable, but involving matchTypes might not always produces the same results as simplified code TypeConvertibleChecker had. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.

[slavapestov]

I'm fine with this. It removes some duplication, and if isConvertible() is wrong we need to fix any problems there.