Use LLVM_USE_SYMLINKS option in install_symlink

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.

Differential Revision: https://reviews.llvm.org/D145443

(cherry picked from commit 5c602c4)

Author: mplatings

llvm/cmake/modules/AddLLVM.cmake

@@ -2191,6 +2191,12 @@ function(llvm_install_library_symlink name dest type)
     set(LLVM_LINK_OR_COPY copy)
   endif()
 
+  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}")