Use LLVM_USE_SYMLINKS option in install_symlink

[michaelplatings]

Summary

The change to potentially use symlinks on Windows was added in
https://reviews.llvm.org/D99170.

LLVM_USE_SYMLINKS was added more recently in
https://reviews.llvm.org/D135578 and allows specifying at configure time
whether or not symlinks should be created. The benefit of using this
option is it allows building the package on a symlink-capable Windows
machine with symlinks disabled so that the resulting package can be used
on a Windows machine that doesn't support symlinks.

[phosek]

LGTM

llvm/cmake/modules/AddLLVM.cmake

function(llvm_install_library_symlink name dest type)
   set(component ${ARG_COMPONENT})
   if(NOT component)
     set(component ${name})
   endif()
 
   set(full_name ${CMAKE_${type}_LIBRARY_PREFIX}${name}${CMAKE_${type}_LIBRARY_SUFFIX})
   set(full_dest ${CMAKE_${type}_LIBRARY_PREFIX}${dest}${CMAKE_${type}_LIBRARY_SUFFIX})
 
+  if(LLVM_USE_SYMLINKS)
+    set(LLVM_LINK_OR_COPY create_symlink)
+  else()
+    set(LLVM_LINK_OR_COPY copy)
+  endif()
+
   set(output_dir lib${LLVM_LIBDIR_SUFFIX})
   if(WIN32 AND "${type}" STREQUAL "SHARED")
     set(output_dir "${CMAKE_INSTALL_BINDIR}")
   endif()
 
   install(SCRIPT ${INSTALL_SYMLINK}
-          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\")"
+          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\" \"${LLVM_LINK_OR_COPY}\")"
           COMPONENT ${component})
 
 endfunction()
 
 function(llvm_install_symlink project name dest)
   get_property(LLVM_DRIVER_TOOLS GLOBAL PROPERTY LLVM_DRIVER_TOOLS)
   if(LLVM_TOOL_LLVM_DRIVER_BUILD
      AND ${dest} IN_LIST LLVM_DRIVER_TOOLS
function(llvm_install_symlink project name dest)
   endif()
 
   set(full_name ${name}${CMAKE_EXECUTABLE_SUFFIX})
   set(full_dest ${dest}${CMAKE_EXECUTABLE_SUFFIX})
   if (${dest} STREQUAL "llvm-driver")
     set(full_dest llvm${CMAKE_EXECUTABLE_SUFFIX})
   endif()
 
+  if(LLVM_USE_SYMLINKS)
+    set(LLVM_LINK_OR_COPY create_symlink)
+  else()
+    set(LLVM_LINK_OR_COPY copy)
+  endif()
+
   set(output_dir "${${project}_TOOLS_INSTALL_DIR}")
 
   install(SCRIPT ${INSTALL_SYMLINK}
-          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\")"
+          CODE "install_symlink(\"${full_name}\" \"${full_dest}\" \"${output_dir}\" \"${LLVM_LINK_OR_COPY}\")"
           COMPONENT ${component})
 
   if (NOT LLVM_ENABLE_IDE AND NOT ARG_ALWAYS_GENERATE)
     add_llvm_install_targets(install-${name}
                              DEPENDS ${name} ${dest}
                              COMPONENT ${name}
                              SYMLINK ${dest})
   endif()

llvm/cmake/modules/LLVMInstallSymlink.cmake

 # We need to execute this script at installation time because the
 # DESTDIR environment variable may be unset at configuration time.
 # See PR8397.
 
 # Set to an arbitrary directory to silence GNUInstallDirs warnings
 # regarding being unable to determine libdir.
 set(CMAKE_INSTALL_LIBDIR "lib")
 include(GNUInstallDirs)
 
-function(install_symlink name target outdir)
+function(install_symlink name target outdir link_or_copy)
+  # link_or_copy is the "command" to pass to cmake -E.
+  # It should be either "create_symlink" or "copy".
+
   set(DESTDIR $ENV{DESTDIR})
   if(NOT IS_ABSOLUTE "${outdir}")
     set(outdir "${CMAKE_INSTALL_PREFIX}/${outdir}")
   endif()
   set(outdir "${DESTDIR}${outdir}")
 
   message(STATUS "Creating ${name}")
 
   execute_process(
-    COMMAND "${CMAKE_COMMAND}" -E create_symlink "${target}" "${name}"
-    WORKING_DIRECTORY "${outdir}" ERROR_VARIABLE has_err)
-  if(CMAKE_HOST_WIN32 AND has_err)
-    execute_process(
-      COMMAND "${CMAKE_COMMAND}" -E copy "${target}" "${name}"
-      WORKING_DIRECTORY "${outdir}")
-  endif()
+    COMMAND "${CMAKE_COMMAND}" -E ${link_or_copy} "${target}" "${name}"
 
 endfunction()