Fix type reference to merged prototype property assignment (#34764)

The constructor function code path in the return type checking of
signatures needs to pass the *merged* symbol of the declaration to
getDeclaredTypeOfClassOrInterface. Other callers of
getDeclaredTypeOfClassOrInterface do this, or used an already-merged
symbol.

Fixes #33993

tests/cases/conformance/salsa/prototypePropertyAssignmentMergedTypeReference.ts

@@ -0,0 +1,14 @@
+// https://github.com/microsoft/TypeScript/issues/33993
+// @noEmit: true
+// @allowJs: true
+// @checkJS: true
+// @filename: prototypePropertyAssignmentMergedTypeReference.js
+var f = function() {
+    return 12;
+};
+
+f.prototype.a = "a";
+
+/** @type {new () => f} */
+var x = f;
+