Deprecate :dllimport and :handle (dafny-lang#3399)

Fixes dafny-lang#2778 and recent discussion: deprecates :dllimport and :handle

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is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

Co-authored-by: davidcok <davidcok@github.com>
Co-authored-by: Fabio Madge <fmadge@amazon.com>
Co-authored-by: Aaron Tomb <aarotomb@amazon.com>
Co-authored-by: Remy Willems <rwillems@amazon.com>
Co-authored-by: Mikaël Mayer <MikaelMayer@users.noreply.github.com>
Co-authored-by: stefan-aws <120379523+stefan-aws@users.noreply.github.com>
Co-authored-by: Aaron Tomb <aarontomb@gmail.com>
Co-authored-by: Alex Chew <alex-chew@users.noreply.github.com>

Source/DafnyCore/Dafny.atg

@@ -120,6 +120,12 @@ void CheckDeclModifiers(ref DeclModifierData dmod, string declCaption, AllowedDe
  }
 }
 
+void CheckAttribute(Errors errors, IToken attr) {
+ if (attr.val == "dllimport" || attr.val == "handle") {
+ errors.Deprecated(attr, $"Attribute :{attr.val} is deprecated and will be removed in Dafny 4.0");
+ }
+}
+
 bool IsAssumeTypeKeyword(IToken la) {
  return la.kind == _assume || la.kind == _assert || la.kind == _expect;
 }
@@ -5175,7 +5181,8 @@ Attribute<ref Attributes attrs>
  AttributeName<out x>
  [ Expressions<args> ]
  "}" (. closeBrace = t; .)
- (. attrs = new UserSuppliedAttributes(x, openBrace, closeBrace, args, attrs);
+ (. CheckAttribute(errors, x);
+ attrs = new UserSuppliedAttributes(x, openBrace, closeBrace, args, attrs);
  attrs.RangeToken = new RangeToken(openBrace, t);
  .)
  .

Test/git-issues/git-issue-2778/git-issue-2778.dfy

@@ -0,0 +1,6 @@
+// RUN: %baredafny resolve --use-basename-for-filename "%s" > "%t"
+// RUN: %diff "%s.expect" "%t"
+
+class {:handle} A {}
+
+method {:dllimport "x"} m() {}

Test/git-issues/git-issue-2778/git-issue-2778.dfy.expect

@@ -0,0 +1,4 @@
+git-issue-2778.dfy(4,8): Warning: Attribute :handle is deprecated and will be removed in Dafny 4.0
+git-issue-2778.dfy(6,9): Warning: Attribute :dllimport is deprecated and will be removed in Dafny 4.0
+
+Dafny program verifier did not attempt verification

docs/dev/news/3399.fix

@@ -0,0 +1 @@
+The attributes :dllimport and :handle are now deprecated. They were undocumented, untested, and not maintained.