[CMake] Improve the clang order-file generation workflow

Summary:
With this change generating clang order files using dtrace uses the following workflow:

cmake <whatever options you want>

ninja generate-order-file

ninja clang

This patch works by setting a default path to the order file (which can be overridden by the user). If the order file doesn't exist during configuration CMake will create an empty one.

CMake then ties up the dependencies between the clang link job and the order file, and generate-order-file overwrites CLANG_ORDER_FILE with the new order file.

Reviewers: bogner

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16896

llvm-svn: 259862