Fix build errors LLVM tests are disabled.

Original patch from alanbaker@google.com

Fixes the error:
CMake Error in <...>/llvm/cmake/modules/CMakeLists.txt:

export called with target "LLVMTestingSupport" which requires target
"gtest" that is not in the export set.

This occurs when LLVM is embedded in a larger project, but is configured not to
include tests. If testing is disabled gtest isn't available and LLVM fails to
configure.

Differential revision: https://reviews.llvm.org/D63097

llvm-svn: 365836