Generate targets for each lit suite.

Summary:
This change makes CMake scan for lit suites and generate a target for each lit test suite. The targets follow the format check-<project>-<suite path>.

For example:
check-llvm-unit - Runs the LLVM unit tests
check-llvm-codegen-arm - Runs the ARM codeine tests

Note: These targets are not generated during multi-configuration generators (i.e. Xcode and Visual Studio) because target clutter impacts UI usability.

Reviewers: chandlerc

Subscribers: aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D8380

llvm-svn: 232671