[Macros] Downgrade "void return for non-expression macro" error in interfaces

This allows us to continue to accept Swift interface files created with
older versions of the Swift 5.9 compiler that emitted a spurious
`-> ()` on non-expression macro declarations.

Author: DougGregor

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2076,10 +2076,21 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     // expression role. Other roles cannot have result types.
     if (auto resultTypeRepr = MD->getResultTypeRepr()) {
       if (!MD->getMacroRoles().contains(MacroRole::Expression)) {
-        auto resultType = MD->getResultInterfaceType();
-        Ctx.Diags.diagnose(
-            MD->arrowLoc, diag::macro_result_type_cannot_be_used, resultType)
-          .highlight(resultTypeRepr->getSourceRange());
+        auto resultType = MD->getResultInterfaceType(); {
+          auto diag = Ctx.Diags.diagnose(
+              MD->arrowLoc, diag::macro_result_type_cannot_be_used, resultType);
+          diag.highlight(resultTypeRepr->getSourceRange());
+
+          // In a .swiftinterface file, downgrade this diagnostic to a warning.
+          // This allows the compiler to process existing .swiftinterface
+          // files that contain this issue.
+          if (resultType->isVoid()) {
+            if (auto sourceFile = MD->getParentSourceFile())
+              if (sourceFile->Kind == SourceFileKind::Interface)
+                diag.limitBehavior(DiagnosticBehavior::Warning);
+          }
+        }
+
         Ctx.Diags.diagnose(MD->arrowLoc, diag::macro_make_freestanding_expression)
           .fixItInsert(MD->getAttributeInsertionLoc(false),
                        "@freestanding(expression)\n");