[lit] Avoid copying llvm/utils/lit/tests/Inputs with lit site configs

Summary:
This is an alternative solution to running the lit test suite on bots
without polluting the source directory. Each input test suite gets an
auto-generated site config in the build directory that points back to
the test input source directory.

This adds some cmake comlexity, but now we don't need to remove and
re-copy the test input directory before every test.

Reviewers: delcypher, modocache

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D36026

llvm-svn: 309602