[ModuleInterface] Don't print SPI attributes on unsupported decls

When emitting the private swiftinterface, the compiler prints the
attribute explicitly even when it is deduced from the context. This can
lead to unparsable private swiftinterface files.

As a narrow fix, check if the decl type is supported before printing the
attribute.

rdar://64039069

Author: xymus

lib/AST/ASTPrinter.cpp

@@ -991,7 +991,9 @@ void PrintAST::printAttributes(const Decl *D) {
     }
 
     // SPI groups
-    if (Options.PrintSPIs) {
+    if (Options.PrintSPIs &&
+        DeclAttribute::canAttributeAppearOnDeclKind(
+          DAK_SPIAccessControl, D->getKind())) {
       interleave(D->getSPIGroups(),
              [&](Identifier spiName) {
                Printer.printAttrName("_spi", true);

test/SPI/private_swiftinterface.swift

@@ -88,6 +88,20 @@ private class PrivateClassLocal {}
   // CHECK-PUBLIC-NOT: extensionSPIMethod
 }
 
+@_spi(LocalSPI) public protocol SPIProto3 {
+// CHECK-PRIVATE: @_spi(LocalSPI) public protocol SPIProto3
+// CHECK-PUBLIC-NOT: SPIProto3
+
+  associatedtype AssociatedType
+  // CHECK-PRIVATE: associatedtype AssociatedType
+  // CHECK-PRIVATE-NOT: @_spi(LocalSPI) associatedtype AssociatedType
+  // CHECK-PUBLIC-NOT: AssociatedType
+
+  func implicitSPIMethod()
+  // CHECK-PRIVATE: @_spi(LocalSPI) func implicitSPIMethod()
+  // CHECK-PUBLIC-NOT: implicitSPIMethod
+}
+
 // Test the dummy conformance printed to replace private types used in
 // conditional conformances. rdar://problem/63352700