[CS] Remove assertion that callee locator has an anchor

When computing type relations of code completion items with respect to the context they are used in, we add a `ConstraintKind::Conversion` constraint with an empty constraint locator. This currently crashes in `hasPreconcurrencyCallee`, but actually having a null anchor doesn’t produce any issues.

Author: ahoppen

lib/Sema/ConstraintSystem.cpp

@@ -481,8 +481,6 @@ ConstraintLocator *ConstraintSystem::getCalleeLocator(
     }
   }
 
-  assert(bool(anchor) && "Expected an anchor!");
-
   {
     // If we have a locator for a member found through key path dynamic member
     // lookup, then we need to chop off the elements after the

validation-test/IDE/crashers_2_fixed/0036-callee-locator-with-null-anchor.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-ide-test --code-completion --code-completion-token COMPLETE --source-filename %s | %FileCheck %s
+
+func foo(completionHandler completion: (@Sendable (Bool) -> Void)? = nil) {}
+
+func test() {
+  foo(completionHandler: #^COMPLETE^#nil)
+}
+
+// CHECK: Begin completions
+// CHECK-DAG: Literal[Nil]/None/TypeRelation[Convertible]: nil[#(@Sendable (Bool) -> Void)?#];
+// CHECK: End completions