Get Swift compiling with MSVC again

[hughbe]

Commit 1

Fixes "error C2678: binary '!=': no operator found which takes a left-hand operand of type 'Serialized' (or there is no acceptable conversion)"
There is an acceptable conversion, MSVC!
https://connect.microsoft.com/VisualStudio/feedback/details/3121202

#include <iostream>

class Decl { };

// Has to be a subclass - using Decl works.
class ValueDecl : public Decl { };

class Serialized {
public:
  /*implicit*/ operator Decl *() const {
    return nullptr;
  }
};

// Has to be const.
void Execute(const ValueDecl *parent) {
  Serialized serialized;

  if (serialized != parent) {
    std::cout << "Hello, World";
  }

  // Replacing the if block with the following is the workaround.
  /*
  Decl *converted = serialized;
  if (converted != parent) {
    std::cout << "Hello, World";
  }*/
}

int main() {
  Execute(nullptr);
}

Commit 2

Fixes warnings "not all control paths return a value"
All control paths are covered, MSVC!
https://connect.microsoft.com/VisualStudio/feedback/details/3121201/msvc-doesnt-detect-a-fully-covered-switch-statement-of-an-enum-class

[hughbe]

@swift-ci please smoke test

[DougGregor]

LGTM, thanks!