[Concurrency] Downgrade isolated call diagnostics when the callee has

an inferred `@preconcurrency` annotation.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -3303,7 +3303,7 @@ namespace {
         } else {
           if (calleeDecl) {
             auto preconcurrency = getContextIsolation().preconcurrency() ||
-            calleeDecl->preconcurrency();
+                getActorIsolation(calleeDecl).preconcurrency();
             ctx.Diags.diagnose(
                                apply->getLoc(), diag::actor_isolated_call_decl,
                                *unsatisfiedIsolation,

test/Concurrency/predates_concurrency.swift

@@ -103,8 +103,8 @@ func testCalls(x: X) {
   let _: () -> Void = onMainActorAlways // expected-complete-tns-warning {{converting function value of type '@MainActor () -> ()' to '() -> Void' loses global actor 'MainActor'}}
 
   // both okay with minimal/targeted... an error with complete.
-  let c = MyModelClass() // expected-complete-tns-error {{call to main actor-isolated initializer 'init()' in a synchronous nonisolated context}}
-  c.f() // expected-complete-tns-error {{call to main actor-isolated instance method 'f()' in a synchronous nonisolated context}}
+  let c = MyModelClass() // expected-complete-tns-warning {{call to main actor-isolated initializer 'init()' in a synchronous nonisolated context}}
+  c.f() // expected-complete-tns-warning {{call to main actor-isolated instance method 'f()' in a synchronous nonisolated context}}
 }
 
 func testCallsWithAsync() async {
@@ -207,3 +207,15 @@ class C { // expected-complete-tns-note {{'C' does not conform to the 'Sendable'
   }
 }
 
+@preconcurrency @MainActor
+class MainActorPreconcurrency {}
+
+class InferMainActorPreconcurrency: MainActorPreconcurrency {
+  static func predatesConcurrency() {}
+  // expected-note@-1 {{calls to static method 'predatesConcurrency()' from outside of its actor context are implicitly asynchronous}}
+}
+
+nonisolated func blah() {
+  InferMainActorPreconcurrency.predatesConcurrency()
+  // expected-warning@-1 {{call to main actor-isolated static method 'predatesConcurrency()' in a synchronous nonisolated context}}
+}