Improve ClangDeclStore ergonomics (#7311)

Change `Lookup` by InstId to return a ClangDecl pointer. All callers
were immediately calling `Get` anyway, so this makes call sites a little
shorter. The other `Lookup` method, by ClangDeclKey, is sometimes called
without calling `Get`, so left that as-is, but renamed to `LookupId`.

Also add a `decl` method to ClangDecl so that the commonly repeated
`clang_decl->key.decl` can be written `clang_decl->decl()`.

toolchain/check/cpp/constant.cpp

@@ -219,13 +219,11 @@ static auto ConvertArgToExpr(Context& context, SemIR::InstId arg_inst_id,
 }
 
 auto EvalCppCall(Context& context, SemIR::LocId loc_id,
-                 SemIR::ClangDeclId clang_decl_id, SemIR::InstBlockId args_id)
+                 const SemIR::ClangDecl& clang_decl, SemIR::InstBlockId args_id)
     -> SemIR::ConstantId {
   const auto& args = context.inst_blocks().Get(args_id);
 
-  auto* decl = context.clang_decls().Get(clang_decl_id).GetAsKey().decl;
-
-  auto* function_decl = cast<clang::FunctionDecl>(decl);
+  auto* function_decl = cast<clang::FunctionDecl>(clang_decl.decl());
 
   // Create expr for the function declaration.
   auto* decl_ref_expr = clang::DeclRefExpr::Create(
@@ -305,10 +303,8 @@ auto MaybeModifyCppThunkCallForConstEval(Context& context, SemIR::Call* call)
 
     function_decl = cast<clang::FunctionDecl>(
         context.clang_decls()
-            .Get(context.clang_decls().Lookup(
-                thunk_callee_function.first_decl_id()))
-            .GetAsKey()
-            .decl);
+            .Lookup(thunk_callee_function.first_decl_id())
+            ->decl());
 
     if (!(function_decl->isConstexpr() || function_decl->isConsteval())) {
       return;