Fix bug #23967. The gtest and gtest_main targets were exported into the

Fix bug #23967. The gtest and gtest_main targets were exported into the
CMake files and should not be by both build systems and also the targets
were also installed by the CMake build system which they should not be.

The problem was that

- the CMake build of LLVM installs and exports the gtest library
targets. We should not being doing this, these are not part of LLVM.

- the Autoconf/Makefile build of LLVM still had gtest libraries in the
installed LLVMConfig.cmake.

These problems would cause problems for an external project because when
calling llvm_map_components_to_libnames(XXX all) ${XXX} would to contain
LLVM's internal gtest libraries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240981 91177308-0d34-0410-b5e6-96231b3b80d8