[cxx-interop] Add a SWIFT_MUTATING macro to <swift/bridging> header

[hyp]

This macro is needed to allow users to annotate constant C++ member functions that do mutate the object, e.g. using a mutable C++ field. This annotation makes Swift import them as mutating methods then.

Note: the compiler already supports it using the swift_attr("mutating") attribute, now we just need a macro. You can use __attribute__((swift_attr("mutating"))) for now to workaround this issue.

[hyp]

Need to land in 5.9 still