[clang-tools-extra] [clangd] Respect llvm_shlib_dir in tests
Add llvm_shlib_dir to variables used in clangd test suite, consistently
to how it is used in the test suites of clang, clang-tools-extra
and a few other components. This is necessary to ensure that
the correct shared libraries are used when building clang standalone --
otherwise, use_clang() sets LD_LIBRARY_PATH to the directory containing
the earlier system installation of clang rather than the just-built
library.
Differential Revision: https://reviews.llvm.org/D135062
(cherry picked from commit
77945a344c3dee3f9735744c8d4151ef2cec6a8d)