[clang-tools-extra] [clangd] Respect llvm_shlib_dir in tests

[mgorny]

Summary

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.

(I'd also like to backport this to 15.x)

[mgorny]

Thanks!

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

 @LIT_SITE_CFG_IN_HEADER@
 
 # Variables needed for common clang config.
 config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
 config.target_triple = "@LLVM_TARGET_TRIPLE@"
 config.host_triple = "@LLVM_HOST_TRIPLE@"
 config.python_executable = "@Python3_EXECUTABLE@"
 # Support substitution of the tools and libs dirs with user parameters. This is
 # used when we can't determine the tool dir at configuration time.
 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@/.."
 config.clangd_build_xpc = @CLANGD_BUILD_XPC@
 config.clangd_enable_remote = @CLANGD_ENABLE_REMOTE@
 config.clangd_tidy_checks = @CLANGD_TIDY_CHECKS@
 config.have_zlib = @LLVM_ENABLE_ZLIB@
 
 # Delegate logic to lit.cfg.py.
 lit_config.load_config(config, "@CMAKE_CURRENT_SOURCE_DIR@/lit.cfg.py")