[cmake] Support running tests in stand-alone builds

Add the CMake bits necessary to run lld tests (and unittests) when
building stand-alone. The code is based on the equivalent code in clang,
and includes:

1. checking for Python, searching for lit and necessary LLVM test tools
(FileCount and not),

2. building LLVM test tools (FileCount and not) from LLVM sources if
they are not installed,

3. building gtest libraries from LLVM sources,

4. adjusting dependencies so that test targets depend only on those LLVM
targets that are available for a particular variant of stand-alone
build.

With this patch, I am able to successfully run 1002 (+10 unsupported)
lit tests on Gentoo using installed LLVM.

Differential Revision: https://reviews.llvm.org/D28750

llvm-svn: 293630