[Wdocumentation] Use the command marker.

Use the proper marker for -Wdocumentation-deprecated-sync instead of
hard-coded the backslash.

Discovered while looking at https://bugs.llvm.org/show_bug.cgi?id=43753

Differential Revision: https://reviews.llvm.org/D71139

clang/include/clang/Basic/DiagnosticCommentKinds.td

@@ -146,8 +146,8 @@ def warn_doc_returns_attached_to_a_void_function : Warning<
 // \deprecated command
 
 def warn_doc_deprecated_not_sync : Warning<
-  "declaration is marked with '\\deprecated' command but does not have "
-  "a deprecation attribute">,
+  "declaration is marked with '%select{\\|@}0deprecated' command but does "
+  "not have a deprecation attribute">,
   InGroup<DocumentationDeprecatedSync>, DefaultIgnore;
 
 def note_add_deprecation_attr : Note<

clang/lib/AST/CommentSema.cpp

@@ -676,9 +676,8 @@ void Sema::checkDeprecatedCommand(const BlockCommandComment *Command) {
       D->hasAttr<UnavailableAttr>())
     return;
 
-  Diag(Command->getLocation(),
-       diag::warn_doc_deprecated_not_sync)
-    << Command->getSourceRange();
+  Diag(Command->getLocation(), diag::warn_doc_deprecated_not_sync)
+      << Command->getSourceRange() << Command->getCommandMarker();
 
   // Try to emit a fixit with a deprecation attribute.
   if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) {

clang/test/Sema/warn-documentation.cpp

@@ -631,9 +631,9 @@ void test_deprecated_4(int a) __attribute__((unavailable));
 /// \deprecated
 void test_deprecated_5(int a);
 
-// expected-warning@+2 {{declaration is marked with '\deprecated' command but does not have a deprecation attribute}} expected-note@+3 {{add a deprecation attribute to the declaration to silence this warning}}
+// expected-warning@+2 {{declaration is marked with '@deprecated' command but does not have a deprecation attribute}} expected-note@+3 {{add a deprecation attribute to the declaration to silence this warning}}
 /// Aaa
-/// \deprecated
+/// @deprecated
 void test_deprecated_6(int a) {
 }