Add specific_id and pattern_inst_id to ClangDecl (#7583)

Exporting class fields in class specifics will require looking up
`ClangDecl`s by the field's `InstId` and the class's `SpecificId`. Add
the `specific_id` to ClangeDecl, and rework the reverse lookup to use a
`Set` with a `KeyContext` rather than a `Map`. The `Lookup` method now
takes an optional `SpecificId` argument, although currently it is always
`None`.

For `VarStorage`, reverse lookup is performed by the pattern `InstId`
rather than the `InstId` of the `VarStorage` itself, so also add
`pattern_inst_id` to `ClangDecl`, and provide a separate
`LookupByPatternInstId` method for reverse lookups. For this lookup, the
`inst_id` part of the key is set to `None`, so only the pattern's
`InstId` is used for lookup.

toolchain/check/cpp/import.cpp

@@ -441,7 +441,12 @@ static auto LookupClangDeclInstId(Context& context, SemIR::ClangDeclKey key)
   const auto& clang_decls = context.clang_decls();
   if (auto context_clang_decl_id = clang_decls.LookupId(key);
       context_clang_decl_id.has_value()) {
-    return clang_decls.Get(context_clang_decl_id).inst_id;
+    const auto& clang_decl = clang_decls.Get(context_clang_decl_id);
+    if (clang_decl.var_storage_inst_id.has_value()) {
+      return clang_decl.var_storage_inst_id;
+    } else {
+      return clang_decls.Get(context_clang_decl_id).inst_id;
+    }
   }
   return SemIR::InstId::None;
 }
@@ -2143,10 +2148,10 @@ static auto ImportVarDecl(Context& context, SemIR::LocId loc_id,
   context.imports().push_back(var_storage_inst_id);
 
   // Register the variable so we don't create it again.
-  context.clang_decls().AddVar({.key = SemIR::ClangDeclKey(var_decl),
-                                .inst_id = var_storage_inst_id,
-                                .is_imported = true},
-                               pattern_id);
+  context.clang_decls().Add({.key = SemIR::ClangDeclKey(var_decl),
+                             .inst_id = pattern_id,
+                             .var_storage_inst_id = var_storage_inst_id,
+                             .is_imported = true});
 
   // Inform Clang that the variable has been referenced.
   context.clang_sema().MarkVariableReferenced(GetCppLocation(context, loc_id),