[AST] @preconcurrency conformance applies to implied conformances as well

A `@preconcurrency` conformance to an inherited protocol should imply
`@preconcurrency` on its parents as well. For example:

```swift
protocol Parent {
  func a()
}

protocol Child: Parent {
  func b()
}

@MainActor
class Test: @preconcurrency Child {
  func a() {
  }

  func b() {
  }
}
```

`Test` conformance to `Parent` implied by its conformance to `Child`
should carry `@preconcurrency` and inject dynamic actor isolation checks
to witness of `a()`.

Resolves: https://github.com/apple/swift/issues/74294
Resolves: rdar://129599097

include/swift/AST/ProtocolConformance.h

@@ -681,7 +681,12 @@ public:
     assert(sourceKind != ConformanceEntryKind::PreMacroExpansion &&
            "cannot create conformance pre-macro-expansion");
     Bits.NormalProtocolConformance.SourceKind = unsigned(sourceKind);
-    ImplyingConformance = implyingConformance;
+    if (auto implying = implyingConformance) {
+      ImplyingConformance = implying;
+      PreconcurrencyLoc = implying->getPreconcurrencyLoc();
+      Bits.NormalProtocolConformance.IsPreconcurrency =
+          implying->isPreconcurrency();
+    }
   }
 
   /// Determine whether this conformance is lazily loaded.