[AST] Add simple LifetimeAnnotation enum.

The enum just provides a convenience around mutually exclusive
attributes which may be applied to decls and to SILFunctionArguments.
This way we can switch over the flavor of annotation that a decl enjoys
rather than having to remember the list of which might affect it, a list
which is likely to grow.

Author: nate-chandler

include/swift/AST/LifetimeAnnotation.h

@@ -0,0 +1,49 @@
+//===--- LifetimeAnnotation.h - Lifetime-affecting attributes ---*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Defines a simple type-safe wrapper around the annotations that affect value
+// lifetimes.  Used for both Decls and SILFunctionArguments.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_AST_LIFETIMEANNOTATION_H
+#define SWIFT_AST_LIFETIMEANNOTATION_H
+
+#include <cstdint>
+
+namespace swift {
+
+/// The annotation on a value (or type) explicitly indicating the lifetime that
+/// it (or its instances) should have.
+///
+/// A LifetimeAnnotation is one of the following three values:
+///
+/// 1) None: No annotation has been applied.
+/// 2) EagerMove: The @_eagerMove attribute has been applied.
+/// 3) Lexical: The @_lexical attribute has been applied.
+struct LifetimeAnnotation {
+  enum Case : uint8_t {
+    None,
+    EagerMove,
+    Lexical,
+  } value;
+
+  LifetimeAnnotation(Case newValue) : value(newValue) {}
+
+  operator Case() const { return value; }
+
+  bool isSome() { return value != None; }
+};
+
+} // namespace swift
+
+#endif