[third-party] Respect LLVM_INSTALL_GTEST for llvm_gtest_main

Pass BUILDTREE_ONLY to llvm_gtest_main only if LLVM_INSTALL_GTEST
is not set.  This fixes 08079863.
Otherwise, llvm_gtest_main cannot be used
in LLVM_DISTRIBUTION_COMPONENTS, effectively making it impossible
to install llvm_gtest correctly.

Differential Revision: https://reviews.llvm.org/D145946