[yaml2obj] Add new command line option `-docnum`.

Input YAML file might contain multiple object file definitions.
New option `-docnum` allows to specify an ordinal number (starting from 1)
of definition used for an object file generation.

Patch reviewed by Sean Silva.

llvm-svn: 209967
diff --git a/llvm/lib/Support/YAMLTraits.cpp b/llvm/lib/Support/YAMLTraits.cpp
index e5f9494..20f8b24 100644
--- a/llvm/lib/Support/YAMLTraits.cpp
+++ b/llvm/lib/Support/YAMLTraits.cpp
@@ -90,8 +90,8 @@
   return false;
 }
 
-void Input::nextDocument() {
-  ++DocIterator;
+bool Input::nextDocument() {
+  return ++DocIterator != Strm->end();
 }
 
 bool Input::mapTag(StringRef Tag, bool Default) {