Roundtrip (export/reimport) class declarations (#7182)

Roundtrip (export/reimport) class declarations

The remapping was previously implemented using name_scopes, which aren't
created for class declarations, only definitions - causing the reimport
to import a fresh copy of the type that mismatched with the original (as
seen in the test baseline).

By changing the mapping to use the reverse part of the clang_decls
mapping this should generalize better (& we probably should further
migrate to that mapping). Though it did trip over some issue with
exactly which instruction is used as the key in the clang_decls map -
this change moves towards standardizing on the first decl id of the
class as its map key.

toolchain/check/testdata/interop/cpp/class/roundtrip.carbon

@@ -41,3 +41,19 @@ auto F(C *p) -> Carbon::CAlias* {
   return p;
 }
 ''';
+
+// --- import.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+class Class1;
+
+inline Cpp '''
+void func(Carbon::Class1* _Nonnull);
+''';
+
+fn Caller(p: Class1*) {
+  Cpp.func(p);
+}