[ASTPrinter] Move LifetimeDescriptor::getString and add test (NFC)

This refactor and test case are preparations for the implementation in
the next commit.

Author: hnrklssn

include/swift/AST/LifetimeDependence.h

@@ -141,17 +141,7 @@ struct LifetimeDescriptor {
     return getName().str() == "immortal";
   }
 
-  std::string getString() const {
-    switch (kind) {
-    case DescriptorKind::Named:
-      return getName().str().str();
-    case DescriptorKind::Ordered:
-      return std::to_string(getIndex());
-    case DescriptorKind::Self:
-      return "self";
-    }
-    llvm_unreachable("Invalid DescriptorKind");
-  }
+  std::string getString() const;
 };
 
 class LifetimeEntry final

lib/AST/LifetimeDependence.cpp

@@ -28,6 +28,18 @@
 
 namespace swift {
 
+std::string LifetimeDescriptor::getString() const {
+  switch (kind) {
+  case DescriptorKind::Named:
+    return getName().str().str();
+  case DescriptorKind::Ordered:
+    return std::to_string(getIndex());
+  case DescriptorKind::Self:
+    return "self";
+  }
+  llvm_unreachable("Invalid DescriptorKind");
+}
+
 LifetimeEntry *
 LifetimeEntry::create(const ASTContext &ctx, SourceLoc startLoc,
                       SourceLoc endLoc, ArrayRef<LifetimeDescriptor> sources,

test/IDE/print_lifetime_attr.swift

@@ -0,0 +1,39 @@
+// REQUIRES: swift_feature_Lifetimes
+// REQUIRES: swift_feature_LifetimeDependence
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-ide-test -enable-experimental-feature Lifetimes -enable-experimental-feature LifetimeDependence -print-swift-file-interface -source-filename %t/test.swift > %t/interface.txt
+// RUN: diff %t/interface.txt %t/interface.txt.expected
+
+//--- test.swift
+public struct S : ~Escapable {
+    @_lifetime(immortal)
+    init() {}
+}
+
+@_lifetime(foo: copy foo)
+public func fooFunc(_ foo: inout S) {}
+
+@_lifetime(&bar)
+public func barFunc(_ bar: inout S) -> S {
+    return bar
+}
+
+@_lifetime(`func`: copy `func`)
+public func funcFunc(func: inout S) {}
+
+//--- interface.txt.expected
+
+public struct S : ~Escapable {
+
+    @_lifetime(immortal)
+    internal init()
+}
+@_lifetime(foo: copy foo)
+public func fooFunc(_ foo: inout S)
+@_lifetime(&bar)
+public func barFunc(_ bar: inout S) -> S
+@_lifetime(func: copy func)
+public func funcFunc(func: inout S)