[cxx-interop] Add test for explicit operator bool

rdar://137879510

Author: egorzhdan

test/Interop/Cxx/stdlib/overlay/Inputs/convertible-to-bool.h

@@ -16,3 +16,11 @@ struct DualOverloadBoolBox {
   operator bool() const { return value; }
   operator bool() { return value; }
 };
+
+struct ExplicitBoolBox {
+private:
+  bool value;
+
+public:
+  explicit operator bool() const { return value; }
+};

test/Interop/Cxx/stdlib/overlay/convertible-to-bool-module-interface.swift

@@ -10,3 +10,6 @@
 
 // CHECK: struct DualOverloadBoolBox : CxxConvertibleToBool {
 // CHECK: }
+
+// CHECK: struct ExplicitBoolBox : CxxConvertibleToBool {
+// CHECK: }