[libclc] Expose prepare_builtins_* variables in top-level CMakeLists (#149657)

Fix `libclc/utils/CMakeLists.txt` to expose `prepare_builtins_*`
variables in parent scope. This was a regression introduced in #148815
where the code was moved into subdirectory, and the variables would no
longer be accessible to calls in top-level CMakeLists, resulting in
attempting to build targets with empty command:

```
[1566/1676] cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
FAILED: clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc
cd /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build && -o /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/clspv--.bc /var/tmp/portage/llvm-core/libclc-22.0.0.9999/work/libclc_build/obj.libclc.dir/clspv--/builtins.opt.clspv--.bc
/bin/sh: line 1: -o: command not found
```

Author: mgorny

libclc/utils/CMakeLists.txt

@@ -12,8 +12,8 @@ set( LLVM_LINK_COMPONENTS
 
 if( LIBCLC_STANDALONE_BUILD )
   add_llvm_executable( prepare_builtins prepare-builtins.cpp )
-  set( prepare_builtins_exe prepare_builtins )
-  set( prepare_builtins_target prepare_builtins )
+  set( prepare_builtins_exe prepare_builtins PARENT_SCOPE )
+  set( prepare_builtins_target prepare_builtins PARENT_SCOPE )
 else()
   add_llvm_utility( prepare_builtins prepare-builtins.cpp )
   setup_host_tool( prepare_builtins PREPARE_BUILTINS prepare_builtins_exe prepare_builtins_target )