[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 77945a3)

Author: mgorny

clang-tools-extra/clangd/test/lit.site.cfg.py.in

@@ -10,6 +10,7 @@ config.python_executable = "@Python3_EXECUTABLE@"
 config.clang_tools_dir = lit_config.substitute("@CURRENT_TOOLS_DIR@")
 config.llvm_tools_dir = lit_config.substitute("@LLVM_TOOLS_DIR@")
 config.llvm_libs_dir = lit_config.substitute("@LLVM_LIBS_DIR@")
+config.llvm_shlib_dir = "@SHLIBDIR@"
 
 config.clangd_source_dir = "@CMAKE_CURRENT_SOURCE_DIR@/.."
 config.clangd_binary_dir = "@CMAKE_CURRENT_BINARY_DIR@/.."