sema: consider decl context with @_spi_available as an SPI export context

This allows chained invocations of multiple @_spi_available decls.

rdar://97748740

Author: nkcsgexi

lib/Sema/TypeCheckAvailability.cpp

@@ -180,7 +180,8 @@ static void forEachOuterDecl(DeclContext *DC, Fn fn) {
 static void computeExportContextBits(ASTContext &Ctx, Decl *D,
                                      bool *spi, bool *implicit, bool *deprecated,
                                      Optional<PlatformKind> *unavailablePlatformKind) {
-  if (D->isSPI())
+  if (D->isSPI() ||
+      D->isAvailableAsSPI())
     *spi = true;
 
   // Defer bodies are desugared to an implicit closure expression. We need to

test/Sema/spi-available-context.swift

@@ -0,0 +1,11 @@
+// REQUIRES: VENDOR=apple
+// REQUIRES: OS=macosx
+// RUN: %target-typecheck-verify-swift -target %target-cpu-apple-macosx11.9 -library-level api
+
+@_spi_available(macOS 10.4, *)
+public protocol Foo { }
+
+@_spi_available(macOS 10.4, *)
+public class Bar {
+    public var foo: Foo?
+}