Move googletest to the third-party directory

This will help improve the project's layering, so that sub-projects
that don't actually need any llvm code can still use googletest
without having to reference code in the llvm directory.

This will also make it easier to consolidate and simplify the standalone
build configurations.

Reviewed By: stellaraccident, lattner, probinson, phosek

Differential Revision: https://reviews.llvm.org/D131919