C++: Fix coroutine IR inconsistencies

While here, remove some dead code related to fixed points from the database
scheme.

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -384,11 +384,23 @@ function_return_type(
  */
 coroutine(
   unique int function: @function ref,
-  int traits: @type ref,
-  int handle: @variable ref,
-  int promise: @variable ref
+  int traits: @type ref
 );
 
+/*
+case @coroutine_placeholder_variable.kind of
+  1 = @handle
+| 2 = @promise
+| 3 = @init_await_resume
+;
+*/
+
+coroutine_placeholder_variable(
+  unique int placeholder_variable: @variable ref,
+  int kind: int ref,
+  int function: @function ref
+)
+
 /** The `new` function used for allocating the coroutine state, if any. */
 coroutine_new(
   unique int function: @function ref,
@@ -829,22 +841,6 @@ variable_template_argument_value(
     int arg_value: @expr ref
 );
 
-/*
-  Fixed point types
-  precision(1) = short, precision(2) = default, precision(3) = long
-  is_unsigned(1) = unsigned is_unsigned(2) = signed
-  is_fract_type(1) = declared with _Fract
-  saturating(1) = declared with _Sat
-*/
-/* TODO
-fixedpointtypes(
-    unique int id: @fixedpointtype,
-    int precision: int ref,
-    int is_unsigned: int ref,
-    int is_fract_type: int ref,
-    int saturating: int ref);
-*/
-
 routinetypes(
     unique int id: @routinetype,
     int return_type: @type ref