SwiftExtract: fix private(set) being ignored for variables with explicit accessors - #908

Fixes #535.

supportedAccessorKinds(binding:minimumAccessLevel:) returned early on
binding.accessorBlock before it reached the private(set) check, so a variable that both
restricts its setter's access level and spells out its accessors explicitly got a setter
binding emitted for a setter it cannot call:

error: cannot assign to property: 'bar' setter is inaccessible
 17 | public func swiftjava_MySwiftLibrary_Foo_bar$set(_ newValue: UnsafePointer<Int8>, _ self: UnsafeRawPointer) {
 18 |   self.assumingMemoryBound(to: Foo.self).pointee.bar = String(cString: newValue)

Moving the modifier check above the accessor block fixes it. The existing
variable_int_privateSet test already covers public private(set) var without an accessor
block, which is why this only shows up when the accessors are written out; the new test
covers that case.

swift test passes, 670 tests in 89 suites. The new test fails before the change and passes
after it.

Not changed here

Subscripts have the same problem by a different path. SwiftAnalysisVisitor.visit(subscriptDecl:)
calls the no-argument accessorBlock.supportedAccessorKinds(), which never consults
minimumAccessLevel, so public private(set) subscript still emits a $set binding. I kept
this PR to the variable case. Happy to fold the subscript fix in here or open a separate
issue for it, whichever you prefer.