[interop][SwiftToCxx] experimentally expose Swift::String type to C++

The only member that's currently exposed is the default initializer, everything else is in extensions

Author: hyp

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -2397,6 +2397,11 @@ static bool isAsyncAlternativeOfOtherDecl(const ValueDecl *VD) {
 }
 
 static bool hasExposeAttr(const ValueDecl *VD) {
+  if (isa<NominalTypeDecl>(VD) && VD->getModuleContext()->isStdlibModule()) {
+    if (VD == VD->getASTContext().getStringDecl())
+      return true;
+    return false;
+  }
   if (VD->getAttrs().hasAttribute<ExposeAttr>())
     return true;
   if (const auto *NMT = dyn_cast<NominalTypeDecl>(VD->getDeclContext()))

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -5,4 +5,15 @@
 // RUN: %check-interop-cxx-header-in-clang(%t/Swift.h -Wno-unused-private-field -Wno-unused-function)
 
 // CHECK: namespace Swift {
+
+// CHECK: class String final {
+// CHECK-NEXT: public:
+// CHECK-NEXT: inline ~String() {
+// CHECK:  }
+// CHECK-NEXT:  inline String(const String &other) {
+// CHECK:  }
+// CHECK-NEXT:  inline String(String &&) = default;
+// CHECK-NEXT:  static inline String init();
+// CHECK-NEXT: private:
+
 // CHECK: } // namespace Swift