Rudimentary support for Doxygen \retval command

[sberg]

Summary

...so that at least a preceding \param etc. that lacks a description gets a -Wdocumentation warning (instead of erroneously treating the \retval ... text as its paragraph).

[gribozavr]

Thanks!

clang/include/clang/AST/CommentCommands.td

 def Invariant  : BlockCommand<"invariant">;
 def Li         : BlockCommand<"li">;
 def Note       : BlockCommand<"note">;
 def Par        : BlockCommand<"par">;
 def Post       : BlockCommand<"post">;
 def Pre        : BlockCommand<"pre">;
 def Remark     : BlockCommand<"remark">;
 def Remarks    : BlockCommand<"remarks">;
+def Retval     : BlockCommand<"retval">;
 def Sa         : BlockCommand<"sa">;
 def See        : BlockCommand<"see">;
 def Since      : BlockCommand<"since">;
 def Todo       : BlockCommand<"todo">;
 def Version    : BlockCommand<"version">;
 def Warning    : BlockCommand<"warning">;
 // HeaderDoc commands
 def Abstract      : BlockCommand<"abstract"> { let IsBriefCommand = 1; }

clang/test/Sema/warn-documentation.cpp

 
 // expected-warning@+4 {{parameter 'x2' is already documented}}
 // expected-note@+2 {{previous documentation}}
 /// \param x1 Aaa.
 /// \param x2 Bbb.
 /// \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);
+
 //===---
 // 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>