Frontend: Reinstate suppression of warnings during interface type-checking.

We're not ready to start emitting warnings when type-checking modules from
interface. It is causing performance regressions and spurious diagnostics to be
emitted.

Reverts a small part of https://github.com/swiftlang/swift/pull/80360.

Resolves rdar://148257136.

test/ModuleInterface/has_storage_attr_bad.swiftinterface

@@ -5,5 +5,5 @@
 // REQUIRES: OS=macosx
 
 extension Array {
-  @_hasStorage public var foo: Int { get set } // expected-warning {{'@_hasStorage' attribute cannot be applied to declaration in extension}}
+  @_hasStorage public var foo: Int { get set }
 }