Sema: Only warn about @inlinable implying @usableFromInline in -swift-version 4.2

Another SwiftNIO request.

Fixes <https://bugs.swift.org/browse/SR-7578>.

Author: slavapestov

lib/Sema/TypeCheckAttr.cpp

@@ -1982,7 +1982,8 @@ void AttributeChecker::visitUsableFromInlineAttr(UsableFromInlineAttr *attr) {
 
   // On internal declarations, @inlinable implies @usableFromInline.
   if (VD->getAttrs().hasAttribute<InlinableAttr>()) {
-    diagnoseAndRemoveAttr(attr, diag::inlinable_implies_usable_from_inline);
+    if (TC.Context.isSwiftVersionAtLeast(4,2))
+      diagnoseAndRemoveAttr(attr, diag::inlinable_implies_usable_from_inline);
     return;
   }
 }

test/Compatibility/attr_inlinable_old_spelling_4.swift

@@ -5,3 +5,6 @@
 
 @_inlineable public func oldInlinableFunction() {}
 @_versioned func oldVersionedFunction() {}
+
+// No warning here
+@_inlineable @_versioned func redundantAttribute() {}

test/Compatibility/attr_inlinable_old_spelling_42.swift

@@ -7,3 +7,6 @@
 
 @_versioned func oldVersionedFunction() {}
 // expected-warning@-1 {{'@_versioned' has been renamed to '@usableFromInline'}}{{2-12=usableFromInline}}
+
+@inlinable @usableFromInline func redundantAttribute() {}
+// expected-warning@-1 {{'@inlinable' declaration is already '@usableFromInline'}}

test/attr/attr_inlinable.swift

@@ -1,7 +1,7 @@
-// RUN: %target-typecheck-verify-swift -swift-version 4
-// RUN: %target-typecheck-verify-swift -swift-version 4 -enable-testing
-// RUN: %target-typecheck-verify-swift -swift-version 4 -enable-resilience
-// RUN: %target-typecheck-verify-swift -swift-version 4 -enable-resilience -enable-testing
+// RUN: %target-typecheck-verify-swift -swift-version 4.2
+// RUN: %target-typecheck-verify-swift -swift-version 4.2 -enable-testing
+// RUN: %target-typecheck-verify-swift -swift-version 4.2 -enable-resilience
+// RUN: %target-typecheck-verify-swift -swift-version 4.2 -enable-resilience -enable-testing
 @inlinable struct TestInlinableStruct {}
 // expected-error@-1 {{'@inlinable' attribute cannot be applied to this declaration}}