[Markup] Add Tag field

You can now specify multiple - Tag: simple fields in documentation
comments for general purpose use. These are not currently intended
to show up in rendered documentation but are for metadata purposes
only.

include/swift/AST/Comment.h

@@ -39,6 +39,10 @@ public:
     return Parts;
   }
 
+  ArrayRef<StringRef> getTags() const {
+    return Parts.Tags;
+  }
+
   Optional<const swift::markup::Paragraph *> getBrief() const {
     return Parts.Brief;
   }

include/swift/Markup/AST.h

@@ -26,6 +26,7 @@ class MarkupASTNode;
 class Paragraph;
 class ParamField;
 class ReturnsField;
+class TagField;
 class ThrowsField;
 
 /// The basic structure of a doc comment attached to a Swift
@@ -36,6 +37,7 @@ struct CommentParts {
   ArrayRef<ParamField *> ParamFields;
   Optional<const ReturnsField *> ReturnsField;
   Optional<const ThrowsField *> ThrowsField;
+  ArrayRef<StringRef> Tags;
 
   bool isEmpty() const {
     return !Brief.hasValue() &&

include/swift/Markup/ASTNodes.def

@@ -72,6 +72,7 @@ MARKUP_AST_NODE(PrivateExtension, MarkupASTNode)
   MARKUP_AST_NODE(RequiresField, PrivateExtension)
   MARKUP_AST_NODE(SeeField, PrivateExtension)
   MARKUP_AST_NODE(SinceField, PrivateExtension)
+  MARKUP_AST_NODE(TagField, PrivateExtension)
   MARKUP_AST_NODE(TODOField, PrivateExtension)
   MARKUP_AST_NODE(ThrowsField, PrivateExtension)
   MARKUP_AST_NODE(VersionField, PrivateExtension)

include/swift/Markup/SimpleFields.def

@@ -43,6 +43,7 @@ MARKUP_SIMPLE_FIELD(ThrowsField, throws, Throws)
 MARKUP_SIMPLE_FIELD(RequiresField, requires, Requires)
 MARKUP_SIMPLE_FIELD(SeeField, seealso, See)
 MARKUP_SIMPLE_FIELD(SinceField, since, Since)
+MARKUP_SIMPLE_FIELD(TagField, tag, Tag)
 MARKUP_SIMPLE_FIELD(TODOField, todo, TODO)
 MARKUP_SIMPLE_FIELD(VersionField, version, Version)
 MARKUP_SIMPLE_FIELD(WarningField, warning, Warning)