Move ObjectYAML code to a new library.

It is only ever used by obj2yaml and yaml2obj. No point in linking it
everywhere.

llvm-svn: 262368
diff --git a/llvm/unittests/CMakeLists.txt b/llvm/unittests/CMakeLists.txt
index 48a573a..49a9b31 100644
--- a/llvm/unittests/CMakeLists.txt
+++ b/llvm/unittests/CMakeLists.txt
@@ -23,6 +23,7 @@
 add_subdirectory(Linker)
 add_subdirectory(MC)
 add_subdirectory(MI)
+add_subdirectory(ObjectYAML)
 add_subdirectory(Option)
 add_subdirectory(ProfileData)
 add_subdirectory(Support)