[Sema] ConstnessChecker: Look through function conversions while checking arguments

In strict concurrency mode some calls could reference a declaration that
is  wrapped in one or more function conversion expressions to apply
concurrency related attributes or erase them (such as `@Sendable` or
`@MainActor`). This shouldn't impact constness checking and the checker
should look through such conversions.

Resolves: rdar://148168219

lib/Sema/ConstantnessSemaDiagnostics.cpp

@@ -175,7 +175,8 @@ static Expr *checkConstantness(Expr *expr) {
       return expr;
 
     ApplyExpr *apply = cast<ApplyExpr>(expr);
-    ValueDecl *calledValue = apply->getCalledValue();
+    ValueDecl *calledValue =
+        apply->getCalledValue(/*skipFunctionConversions=*/true);
     if (!calledValue)
       return expr;
 
@@ -300,7 +301,8 @@ static void diagnoseConstantArgumentRequirementOfCall(const CallExpr *callExpr,
                                                       const ASTContext &ctx) {
   assert(callExpr && callExpr->getType() &&
          "callExpr should have a valid type");
-  ValueDecl *calledDecl = callExpr->getCalledValue();
+  ValueDecl *calledDecl =
+      callExpr->getCalledValue(/*skipFunctionConversions=*/true);
   if (!calledDecl || !isa<AbstractFunctionDecl>(calledDecl))
     return;
   AbstractFunctionDecl *callee = cast<AbstractFunctionDecl>(calledDecl);

test/Sema/diag_constantness_check_os_log_swift6.swift

@@ -0,0 +1,33 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+// REQUIRES: VENDOR=apple
+// REQUIRES: concurrency
+
+import OSLogTestHelper
+
+class Log {
+  @_semantics("oslog.requires_constant_arguments")
+  func debug(_: OSLogMessage) {}
+}
+
+@_semantics("constant_evaluable")
+@_transparent
+public func __outputFormatter(_ key: String, fallback: OSLogMessage) -> OSLogMessage {
+    fallback
+}
+
+@MainActor
+@_semantics("constant_evaluable")
+@_transparent
+public func __isolatedOutputFormatter(_ key: String, fallback: OSLogMessage) -> OSLogMessage {
+    fallback
+}
+
+func test(log: Log) {
+  log.debug(__outputFormatter("1", fallback: "msg")) // Ok
+}
+
+@MainActor
+func isolatedTest(log: Log) {
+  log.debug(__isolatedOutputFormatter("1", fallback: "msg")) // Ok
+}