Value initialise the const member of TranslatePass.

Clang rejects a const member without an initialiser when Flow::Block has no
user provided constructor.

--- a/src/shader_recompiler/frontend/maxwell/structured_control_flow.cpp
+++ b/src/shader_recompiler/frontend/maxwell/structured_control_flow.cpp
@@ -976,7 +976,7 @@
     Environment& env;
     IR::AbstractSyntaxList& syntax_list;
     bool uses_demote_to_helper{};
-    const Flow::Block dummy_flow_block;
+    const Flow::Block dummy_flow_block{};
 };
 } // Anonymous namespace