[LIT] Add support for `UNSUPPORTED` tag to `TestRunner.parseIntegratedTestScript`

Summary:
This patch gives me just enough to leverage the existing functionality in `TestRunner` for use in `libc++` and `libc++abi` .

It does the following:
* Adds the `UNSUPPORTED` tag to `TestRunner.parseIntegratedTestScript`.
* Allows `parseIntegratedTestScript` to return an empty script if a script is not required by the caller.



Reviewers: ddunbar, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits, llvm-commits

Differential Revision: http://reviews.llvm.org/D6589