[libclc] Fix installed symlinks to be relative again (llvm#149728)

Fix the symlink creation logic to use relative paths instead of
absolute, in order to ensure that the installed symlinks actually refer
to the installed .bc files rather than the ones from the build
directory. This was broken in llvm#146833. The change is a bit roundabout
but it attempts to preserve the spirit of llvm#146833, that is the ability
to use multiple output directories (provided they all resides in
`${LIBCLC_OUTPUT_LIBRARY_DIR}` and preserve the same structure in the
installed tree).

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

libclc/cmake/modules/AddLibclc.cmake

@@ -425,17 +425,21 @@ function(add_libclc_builtin_set)
       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} )
   endif()
 
-  if(CMAKE_HOST_UNIX OR LLVM_USE_SYMLINKS)
-    set(LIBCLC_LINK_OR_COPY create_symlink)
-  else()
-    set(LIBCLC_LINK_OR_COPY copy)
-  endif()
-
   foreach( a IN LISTS ARG_ALIASES )
+    if(CMAKE_HOST_UNIX OR LLVM_USE_SYMLINKS)
+      cmake_path(RELATIVE_PATH libclc_builtins_lib
+        BASE_DIRECTORY ${LIBCLC_OUTPUT_LIBRARY_DIR}
+        OUTPUT_VARIABLE LIBCLC_LINK_OR_COPY_SOURCE)
+      set(LIBCLC_LINK_OR_COPY create_symlink)
+    else()
+      set(LIBCLC_LINK_OR_COPY_SOURCE ${libclc_builtins_lib})
+      set(LIBCLC_LINK_OR_COPY copy)
+    endif()
+
     set( alias_suffix "${a}-${ARG_TRIPLE}.bc" )
     add_custom_command(
       OUTPUT ${LIBCLC_OUTPUT_LIBRARY_DIR}/${alias_suffix}
-      COMMAND ${CMAKE_COMMAND} -E ${LIBCLC_LINK_OR_COPY} ${libclc_builtins_lib} ${LIBCLC_OUTPUT_LIBRARY_DIR}/${alias_suffix}
+      COMMAND ${CMAKE_COMMAND} -E ${LIBCLC_LINK_OR_COPY} ${LIBCLC_LINK_OR_COPY_SOURCE} ${LIBCLC_OUTPUT_LIBRARY_DIR}/${alias_suffix}
       DEPENDS prepare-${obj_suffix}
     )
     add_custom_target( alias-${alias_suffix} ALL