Fix small explicit module build test file issues

- Explicitly import `Swift` in swiftinterface files
- Use quote header file imports instead of <>

Author: artemcm

TestInputs/ExplicitModuleBuilds/CHeaders/B.h

@@ -1,4 +1,4 @@
-#include <A.h>
+#include "A.h"
 
 void funcB(void);
 

TestInputs/ExplicitModuleBuilds/CHeaders/C.h

@@ -1,3 +1,3 @@
-#include <B.h>
+#include "B.h"
 
 void funcC(void);

TestInputs/ExplicitModuleBuilds/Swift/A.swiftinterface

@@ -1,5 +1,6 @@
 // swift-interface-format-version: 1.0
 // swift-module-flags: -module-name A
+import Swift
 @_exported import A
 public func overlayFuncA() { }
 

TestInputs/ExplicitModuleBuilds/Swift/G.swiftinterface

@@ -2,7 +2,7 @@
 // swift-module-flags: -module-name G -swift-version 5
 
 #if swift(>=5.0)
-
+import Swift
 @_exported import G
 public func overlayFuncG() { }