Reverse Interop: Class declarations (#6955)

Generate class declarations for Carbon classes referenced from C++

Based on #6940, review from eb62070a01
onwards

toolchain/check/testdata/interop/cpp/reverse/simple.carbon

@@ -14,6 +14,8 @@
 package Other;
 namespace Nested;
 namespace Nested.Again;
+
+class Class1;
 // --- namespace.carbon
 
 library "[[@TEST_NAME]]";
@@ -23,4 +25,14 @@ import Cpp inline '''
 namespace X = Carbon::Other;
 namespace Y = Carbon::Other::Nested;
 namespace Z = Carbon::Other::Nested::Again;
+Carbon::Other::Class1 *glbl;
+
+template<typename T> void f1() {
+  T* v;
+}
+
+void f2() {
+  f1<Carbon::Other::Class1>();
+}
+
 ''';