[unittest] Allow LLVM unit test to run under a wrapper program.

[zeroomega]

This patch add CMake option "LLVM_GTEST_RUN_UNDER" to LLVM unittest configuration. When specified, LLVM unittest will be run under the wrapper program specified by this option. This feature can simplify the setup to run LLVM unittest on a target platform that is different than host.

[zeroomega]

I am a bit hesitated if I should use LLVM_GTEST_RUN_UNDER to eventually allow all gtest based unit test can be redirected to a wrapper program or treat each component individually to just use "LLVM_GTEST_UNITTEST_RUN_UNDER" for this patch. We can add "LLVM_GTEST_CLANG_TEST_RUN_UNDER" later for clang and "LLVM_GTEST_CLANG_TOOLS_TEST_RUN_UNDER" for clang-tools.

In the use case we have, we want to run googletest based unittest on a cross compiled target platform. In this case, all gtest based unittest should be invoked under the wrapper program, so a global LLVM_GTEST_RUN_UNDER options make more sense.