[cmake] Build gtest from LLVM when building standalone

Include the gtest utility directory from LLVM sources when performing
a stand-alone build of LLDB. This is necessary to have a correct gtest
library to link tests against, as the one used by LLVM is not installed
(and not supposed to be). This is the same approach as used in clang.

Differential Revision: https://reviews.llvm.org/D26245

llvm-svn: 285865