[CS] Allow InsertExplicitCall for higher-order functions

Allow `InsertExplicitCall` for e.g converting
`() -> () -> Void` to `() -> Void`. No test since
it's already covered in the test suite when the
next commit is applied.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -5186,11 +5186,16 @@ bool ConstraintSystem::repairFailures(
     // side isn't, let's check it would be possible to fix
     // this by forming an explicit call.
     auto convertTo = dstType->lookThroughAllOptionalTypes();
-    // Right-hand side can't be - a function, a type variable or dependent
-    // member, or `Any` (if function conversion to `Any` didn't succeed there
-    // is something else going on e.g. problem with escapiness).
-    if (convertTo->is<FunctionType>() || convertTo->isTypeVariableOrMember() ||
-        convertTo->isAny())
+
+    // If the RHS is a function type, the source must be a function-returning
+    // function.
+    if (convertTo->is<FunctionType>() && !resultType->is<FunctionType>())
+      return false;
+
+    // Right-hand side can't be a type variable or dependent member, or `Any`
+    // (if function conversion to `Any` didn't succeed there is something else
+    // going on e.g. problem with escapiness).
+    if (convertTo->isTypeVariableOrMember() || convertTo->isAny())
       return false;
 
     ConstraintKind matchKind;