[libclc] Change libclc install dir to ${clang_resource_dir}/lib/libclc in in-tree build

[wenju-he]

Commit df74736 placed libclc libraries into clang resource dir /lib/libclc at build stage.
This PR does it at install stage as well.
Note that in standalone (not in-tree) build, libclc is still installed to old ${CMAKE_INSTALL_DATADIR}/clc dir.

[Copilot]

Pull Request Overview

Adjust installation path for libclc so that in an in-tree build the libraries install into Clang's resource directory (lib/libclc), matching prior build-stage behavior; standalone build keeps the old datadir path.

• Introduces LIBCLC_INSTALL_DIR variable for both standalone and in-tree configurations.
• Reworks path construction for in-tree build using get_clang_resource_dir and cmake_path commands.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
libclc/cmake/modules/AddLibclc.cmake	Switch install destination to use computed LIBCLC_INSTALL_DIR variable.
libclc/CMakeLists.txt	Define LIBCLC_INSTALL_DIR and alter in-tree output and install path logic using get_clang_resource_dir and cmake_path.

[Copilot]

Appending an (likely) absolute path ${LIBCLC_INSTALL_DIR} (after line 107) to LIBCLC_OUTPUT_LIBRARY_DIR (line 109) can discard the earlier parent path per cmake_path APPEND semantics when an absolute component is used, resulting in LIBCLC_OUTPUT_LIBRARY_DIR incorrectly mirroring the install directory. If the intent is to place build artifacts under the build tree while installing to the resource dir, compute a separate resource path (e.g., get_clang_resource_dir with PREFIX for build output) or append only relative components (lib, libclc) instead of the full absolute path.

Suggested change

	cmake_path( APPEND LIBCLC_OUTPUT_LIBRARY_DIR ${LIBCLC_INSTALL_DIR} )
	cmake_path( APPEND LIBCLC_OUTPUT_LIBRARY_DIR "lib" "libclc" )

[wenju-he]

Appending an (likely) absolute path ${LIBCLC_INSTALL_DIR}

not true, get_clang_resource_dir always returns a relative path