Documentation: convert WritingAnLLVMPass.html to reST.

Patch by Anthony Mykhailenko with small fixes by me.

llvm-svn: 169992
diff --git a/llvm/docs/CMake.rst b/llvm/docs/CMake.rst
index 9bf0d56..f895788 100644
--- a/llvm/docs/CMake.rst
+++ b/llvm/docs/CMake.rst
@@ -355,6 +355,8 @@
 
   target_link_libraries(mycompiler ${REQ_LLVM_LIBRARIES})
 
+.. _cmake-out-of-source-pass:
+
 Developing LLVM pass out of source
 ----------------------------------