AST: fix missing header files

`ForeignAsyncConvention.h` and `ForeignErrorConvention.h` must be included in `Decl.h`, because those types are used in an `llvm::Optional` in `Decl.h`.

Author: eeckstein

include/swift/AST/Decl.h

@@ -26,6 +26,8 @@
 #include "swift/AST/DefaultArgumentKind.h"
 #include "swift/AST/DiagnosticConsumer.h"
 #include "swift/AST/DiagnosticEngine.h"
+#include "swift/AST/ForeignAsyncConvention.h"
+#include "swift/AST/ForeignErrorConvention.h"
 #include "swift/AST/FreestandingMacroExpansion.h"
 #include "swift/AST/GenericParamKey.h"
 #include "swift/AST/IfConfigClause.h"
@@ -72,8 +74,6 @@ namespace swift {
   struct ExternalSourceLocs;
   class CaptureListExpr;
   class DeclRefExpr;
-  class ForeignAsyncConvention;
-  class ForeignErrorConvention;
   class LiteralExpr;
   class BraceStmt;
   class DeclAttributes;

lib/AST/Decl.cpp

@@ -27,8 +27,6 @@
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Expr.h"
-#include "swift/AST/ForeignAsyncConvention.h"
-#include "swift/AST/ForeignErrorConvention.h"
 #include "swift/AST/GenericEnvironment.h"
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/Initializer.h"