Filter libraries that are not installed out of CMake exports (currently

gtest and gtest_main) when generating ``Makefile.llvmbuild``.

Libraries that are not installed should not be exported because they
won't be available from an install tree.  Rather than filtering out the
gtest libraries in cmake/modules/Makefile, simply teach llvm-build to
filter out libraries that will not be installed from its generated list
of exported libraries.

Note that LLVMBUILD_LIB_DEPS_* are used during our own CMake build
process so we cannot filter LLVMBUILD_LIB_DEPS_gtest* out in llvm-build.
We must leave this gtest filter logic in cmake/modules/Makefile.

llvm-svn: 245718