[AutoDiff] Rename `@transposing` to `@transpose(of:)`. (#28488)
Rename `@transposing` to `@transpose(of:)`. `@transpose(of:)` more clearly
evokes transpose registration; the syntax is otherwise unchanged.
Discussed here:
https://github.com/apple/swift/pull/28321#discussion_r347757647
Remove `@transposing`. Removal without deprecation should be fine because
there are no known users of `@transposing` attribute.
Resolves TF-992.
TF-1009 tracks `@transpose` syntax support for qualified names.
