[CMake] try creating symlink first on windows

//-E create_symlink//  is available on windows since CMake 3.13 (LLVM now uses 3.13.4)
It may needs administrator privileges or enabled developer mode (Windows 10)
See https://cmake.org/cmake/help/latest/release/3.13.html

Reviewed By: kbobyrev

Differential Revision: https://reviews.llvm.org/D99170

Author: kirillbobyrev

llvm/cmake/modules/LLVMInstallSymlink.cmake

@@ -4,18 +4,17 @@
 
 function(install_symlink name target outdir)
   set(DESTDIR $ENV{DESTDIR})
-  if(CMAKE_HOST_UNIX)
-    set(LINK_OR_COPY create_symlink)
-  else()
-    set(LINK_OR_COPY copy)
-  endif()
-
   set(bindir "${DESTDIR}${CMAKE_INSTALL_PREFIX}/${outdir}/")
 
   message(STATUS "Creating ${name}")
 
   execute_process(
-    COMMAND "${CMAKE_COMMAND}" -E ${LINK_OR_COPY} "${target}" "${name}"
-    WORKING_DIRECTORY "${bindir}")
+    COMMAND "${CMAKE_COMMAND}" -E create_symlink "${target}" "${name}"
+    WORKING_DIRECTORY "${bindir}" ERROR_VARIABLE has_err)
+  if(CMAKE_HOST_WIN32 AND has_err)
+    execute_process(
+      COMMAND "${CMAKE_COMMAND}" -E copy "${target}" "${name}"
+      WORKING_DIRECTORY "${bindir}")
+  endif()
 
 endfunction()