Comment parsing: Treat \ref as inline command

[aaronpuchert]

Summary

It's basically Doxygen's version of a link and can happen anywhere
inside of a paragraph. Fixes a bogus warning about empty paragraphs when
a parameter description starts with a link.

[gribozavr2]

Thanks!

clang/include/clang/AST/CommentCommands.td

 //===----------------------------------------------------------------------===//
 
 def B      : InlineCommand<"b">;
 def C      : InlineCommand<"c">;
 def P      : InlineCommand<"p">;
 def A      : InlineCommand<"a">;
 def E      : InlineCommand<"e">;
 def Em     : InlineCommand<"em">;
+def Ref    : InlineCommand<"ref">;
 def Anchor : InlineCommand<"anchor">;
 
 //===----------------------------------------------------------------------===//
 // BlockCommand
 //===----------------------------------------------------------------------===//
 
 def Brief : BlockCommand<"brief"> { let IsBriefCommand = 1; }
 def Short : BlockCommand<"short"> { let IsBriefCommand = 1; }

 
 def Section       : VerbatimLineCommand<"section">;
 def Subsection    : VerbatimLineCommand<"subsection">;
 def Subsubsection : VerbatimLineCommand<"subsubsection">;
 def Paragraph     : VerbatimLineCommand<"paragraph">;
 
 def Mainpage : VerbatimLineCommand<"mainpage">;
 def Subpage  : VerbatimLineCommand<"subpage">;
-def Ref      : VerbatimLineCommand<"ref">;
 
 def Relates     : VerbatimLineCommand<"relates">;
 def Related     : VerbatimLineCommand<"related">;
 def RelatesAlso : VerbatimLineCommand<"relatesalso">;
 def RelatedAlso : VerbatimLineCommand<"relatedalso">;
 
 //===----------------------------------------------------------------------===//
 // DeclarationVerbatimLineCommand

clang/test/Sema/warn-documentation.cpp

 /// \param x2 Ccc.
 int test_param22(int x1, int x2, int x3);
 
 // expected-warning@+1 {{empty paragraph passed to '\param' command}}
 /// \param a
 /// \retval 0 Blah blah.
 int test_param23(int a);
 
+/// \param a \ref test_param23 has an empty paragraph, this doesn't.
+int test_param24(int a);
+
 //===---
 // Test that we treat typedefs to some non-function types as functions for the
 // purposes of documentation comment parsing.
 //===---
 
 namespace foo {
   inline namespace bar {
     template<typename>