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This introduces support for converting a Swift closure that captures variables from its surrounding context into an instance of `std::function`, which is useful for working with C++ APIs that use callbacks. Each instantiation of `std::function` gets a synthesized Swift constructor that takes a Swift closure. Unlike the previous implementation, the closure is _not_ marked as `@convention(c)`. The body of the constructor is created lazily. Under the hood, the closure is bitcast to a pair of a function pointer and a context pointer, which are then wrapped in a C++ object, `__SwiftFunctionWrapper`, that manages the lifetime of the context object via calls to `swift_retain`/`swift_release` from the copy constructor and the destructor. The `__SwiftFunctionWrapper` class is templated, and is instantiated by ClangImporter. rdar://133777029