Comment parsing: add CommentOptions to allow specifying custom comment block commands

Add an ability to specify custom documentation block comment commands via a new
class CommentOptions.  The intention is that this class will hold future
customizations for comment parsing, including defining documentation comments
with specific numbers of parameters, etc.

CommentOptions instance is a member of LangOptions.

CommentOptions is controlled by a new command-line parameter
-fcomment-block-commands=Foo,Bar,Baz.

llvm-svn: 175892
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index a9df146..514c76e 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -654,7 +654,7 @@
     DeclarationNames(*this),
     ExternalSource(0), Listener(0),
     Comments(SM), CommentsLoaded(false),
-    CommentCommandTraits(BumpAlloc),
+    CommentCommandTraits(BumpAlloc, LOpts.CommentOpts),
     LastSDM(0, 0),
     UniqueBlockByRefTypeID(0)
 {