Add 'throws' keyword to doc comments

Adding the following to a doc comment:

- throws: ...

Will create a description about what/when the function will throw.
This should be a peer to "- returns:" and "- parameter:" and not appear
inline in the description.

rdar://problem/21621679

Swift SVN r29831

lib/AST/DocComment.cpp

@@ -261,6 +261,8 @@ bool extractSimpleField(llvm::markup::MarkupContext &MC,
 
     if (auto RF = dyn_cast<llvm::markup::ReturnsField>(Field))
       Parts.ReturnsField = RF;
+    else if (auto TF = dyn_cast<llvm::markup::ThrowsField>(Field))
+      Parts.ThrowsField = TF;
     else
       Parts.BodyNodes.push_back(Field);
   }