Sema: Limit the warning about @_implementationOnly imports deprecation to Swift targets

Preserve the warning to use `internal import` instead of `@_implementationOnly`
to imports of Swift modules only. This warning can be noisy, limiting it may
prevent users from outright learning to ignore it. Typical uses of an
`@_implementationOnly` import of a clang module is often for a project
internal module instead of a layering concern as we have with Swift module
targets. We can leave legacy uses of `@_implementationOnly` in peace for now.

Author: xymus

lib/Sema/ImportResolution.cpp

@@ -818,11 +818,13 @@ void UnboundImport::validateResilience(NullablePtr<ModuleDecl> topLevelModule,
       import.implementationOnlyRange.isValid()) {
     if (SF.getParentModule()->isResilient()) {
       // Encourage replacing `@_implementationOnly` with `internal import`.
-      auto inFlight =
-        ctx.Diags.diagnose(import.importLoc,
-                           diag::implementation_only_deprecated);
-      inFlight.fixItReplace(import.implementationOnlyRange, "internal");
-    } else if ( // Non-resilient
+      if (!topLevelModule.get()->isNonSwiftModule()) {
+        auto inFlight =
+          ctx.Diags.diagnose(import.importLoc,
+                             diag::implementation_only_deprecated);
+        inFlight.fixItReplace(import.implementationOnlyRange, "internal");
+      }
+    } else if ( // Non-resilient client
       !(((targetName.str() == "CCryptoBoringSSL" ||
           targetName.str() == "CCryptoBoringSSLShims") &&
          (importerName.str() == "Crypto" ||

test/Sema/implementation-only-import-from-non-resilient.swift

@@ -53,9 +53,7 @@ import ClangModuleB
 import SwiftModuleB
 
 @_implementationOnly import ClangModuleA
-// expected-warning @-1 {{'@_implementationOnly' is deprecated, use 'internal import' instead}}
 @_implementationOnly import ClangModuleA.Submodule
-// expected-warning @-1 {{'@_implementationOnly' is deprecated, use 'internal import' instead}}
 import ClangModuleB
 
 //--- Crypto.swift