swift234

Revert "Sema: Always synthesize accessors for structs, unless they were imported from Clang"

This reverts commit 2b6ab633fc8a04b76ebeceba0e246a86e76b28dc because it
at least breaks:
    Swift :: stdlib/SequenceType.swift.gyb
and possibly also results in some or all of these failures:
    Swift :: compiler_crashers/27944-swift-astvisitor.swift
    Swift :: compiler_crashers/28200-swift-typebase-getdesugaredtype.swift
    Swift :: stdlib/CollectionType.swift.gyb
    Swift :: stdlib/MicroStdlib.swift