Start with writing failing tests

Author: Huddie

test/Interop/Cxx/class/method/Inputs/methods.h

@@ -25,6 +25,9 @@ struct HasMethods {
 
   NonTrivialInWrapper nonConstPassThroughAsWrapper(int a) { return {a}; }
   NonTrivialInWrapper constPassThroughAsWrapper(int a) const { return {a}; }
+
+  int increment(int a) { return a + 1; }
+  int increment(int a) const { return a + 1; }
 };
 
 #endif // TEST_INTEROP_CXX_CLASS_METHOD_METHODS_H

test/Interop/Cxx/class/method/methods-module-interface.swift

@@ -17,3 +17,9 @@
 
 // CHECK: mutating func nonConstPassThroughAsWrapper(_ a: Int32) -> NonTrivialInWrapper
 // CHECK: func constPassThroughAsWrapper(_ a: Int32) -> NonTrivialInWrapper
+
+// CHECK: mutating func incrementMutable(_ a: Int32) -> Int32
+// CHECK: func incrementMutable(_ a: Int32) -> Int32
+
+// CHECK: const func increment(_ a: Int32) -> Int32
+// CHECK: func increment(_ a: Int32) -> Int32