[TypeChecker] Make sure that @preconcurrency use is detected in assignments

If destination is marked as `@preconcurrency` the Sendable conformance
errors should be downgraded to warnings even in Swift 6 mode just like
for member and subscript references.

Author: xedin

lib/Sema/TypeCheckAvailability.cpp

@@ -3671,6 +3671,11 @@ class ExprAvailabilityWalker : public BaseDiagnosticWalker {
       maybeDiagKeyPath(KP);
     }
     if (auto A = dyn_cast<AssignExpr>(E)) {
+      // Attempting to assign to a @preconcurrency declaration should
+      // downgrade Sendable conformance mismatches to warnings.
+      PreconcurrencyCalleeStack.push_back(
+        hasReferenceToPreconcurrencyDecl(A->getDest()));
+
       walkAssignExpr(A);
       return Action::SkipChildren(E);
     }

test/Concurrency/predates_concurrency_swift6.swift

@@ -307,4 +307,7 @@ do {
     // expected-warning@-1 {{type '() -> ()' does not conform to the 'Sendable' protocol}}
     // expected-note@-2 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   }
+
+  // If destination is @preconcurrency the Sendable conformance error should be downgraded
+  d = data // expected-warning {{type 'Any' does not conform to the 'Sendable' protocol}}
 }