[Concurrency] Allow properties with @Sendable function types witness non-Sendable requirements

This is already supported for method and subscript witnesses that get
their types decomposed.

```
protocol P {
  var test: () -> Void { get }
}

extension S : P {
  let test: @sendable () -> Void // no concurrency warnings
}
```

Resolves: rdar://133403333

Author: xedin

lib/Sema/TypeCheckProtocol.cpp

@@ -885,7 +885,25 @@ RequirementMatch swift::matchWitness(
     if (!req->isObjC() && reqTypeIsIUO != witnessTypeIsIUO)
       return RequirementMatch(witness, MatchKind::TypeConflict, witnessType);
 
-    if (auto result = matchTypes(std::get<0>(types), std::get<1>(types))) {
+    auto reqType = std::get<0>(types);
+    auto witnessType = std::get<1>(types);
+
+    // Allow witnesses with @Sendable types to match non-Sendable requirements.
+    // This is already supported for method and subscript witnesses that get
+    // their types decomposed.
+    if (auto *reqFnType = reqType->getAs<AnyFunctionType>()) {
+      if (auto *witnessFnType = witnessType->getAs<AnyFunctionType>()) {
+        if (reqFnType->hasExtInfo() && witnessFnType->hasExtInfo()) {
+          auto reqExtInfo = reqFnType->getExtInfo();
+          auto witnessExtInfo = witnessFnType->getExtInfo();
+
+          if (!reqExtInfo.isSendable() && witnessExtInfo.isSendable())
+            reqType = reqFnType->withExtInfo(reqExtInfo.withSendable());
+        }
+      }
+    }
+
+    if (auto result = matchTypes(reqType, witnessType)) {
       return std::move(result.value());
     }
   }

test/Concurrency/sendable_conformance_checking.swift

@@ -187,3 +187,20 @@ actor MyActor {
   // expected-warning@+1 {{non-final class 'Nested' cannot conform to 'Sendable'; use '@unchecked Sendable'; this is an error in the Swift 6 language mode}}
   class Nested: Sendable {}
 }
+
+@Sendable func globalFn() {}
+
+protocol NoSendableReqs {
+  var prop: () -> Void { get }
+  static var staticProp: () -> Void { get }
+}
+
+public struct TestSendableWitnesses1 : NoSendableReqs {
+  var prop: @Sendable () -> Void // Ok (no warnings)
+  static let staticProp: @Sendable () -> Void = { } // Ok (no warnings)
+}
+
+public struct TestSendableWitnesses2 : NoSendableReqs {
+  var prop = globalFn // Ok (no warnings)
+  static let staticProp = globalFn // Ok (no warnings)
+}