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/export.cpp

@@ -1439,10 +1439,10 @@ auto ExportVarToCpp(Context& context, SemIR::InstId inst_id,
       context.ast_context(), decl_context,
       /*StartLoc=*/clang_loc, /*IdLoc=*/clang_loc, identifier_info, cpp_type,
       /*TInfo=*/nullptr, clang::SC_Extern);
-  context.clang_decls().AddVar(
+  context.clang_decls().Add(
       {.key = SemIR::ClangDeclKey::ForNonFunctionDecl(var_decl),
-       .inst_id = inst_id},
-      var_storage.pattern_id);
+       .inst_id = var_storage.pattern_id,
+       .var_storage_inst_id = inst_id});
 
   if (scope_inst.Is<SemIR::ClassDecl>()) {
     SetCppClassMemberAccess(name_scope, entity_name.name_id, var_decl);