C++: Fix coroutine IR inconsistencies

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

cpp/downgrades/7ff6a6e53dbcff09d1b9b758b594bc6d17366863/coroutine.ql

@@ -0,0 +1,18 @@
+ class Function extends @function {
+  string toString() { none() }
+}
+
+class Type extends @type {
+  string toString() { none() }
+}
+
+class Variable extends @variable {
+  string toString() { none() }
+}
+
+from Function func, Type traits, Variable handle, Variable promise
+where
+  coroutine(func, traits) and
+  coroutine_placeholder_variable(handle, 1, func) and
+  coroutine_placeholder_variable(promise, 2, func)
+select func, traits, handle, promise

cpp/downgrades/7ff6a6e53dbcff09d1b9b758b594bc6d17366863/upgrade.properties

@@ -0,0 +1,4 @@
+description: Improve handling of coroutine placeholder variables
+compatibility: full
+coroutine.rel: run coroutine.qlo
+coroutine_placeholder_variable.rel: delete