[gn build] Use llvm-ar when clang_base_path is specified

[aeubanks]

Summary

This prevents warnings with use_thinlto like

bfd plugin: LLVM gold plugin has failed to create LTO module: Not an int attribute (Producer: 'LLVM15.0.0git' Reader: 'LLVM 13.0.1')

[thakis]

I'm guessing we have the same problem on mac, but fine to fix that once someone tries to build with LTO there.

Can you say "linux" or "non-mac non-win" or similar in the commit message? Else, lg.

(Maybe we only want this if && use_thinlto? But as-is is fine too.)

llvm/utils/gn/build/toolchain/BUILD.gn

tool("stamp") {
       command = "touch {{output}}"
       description = "STAMP {{output}}"
     }
   }
 }
 
 unix_toolchain("unix") {
   if (current_os != "ios" && current_os != "mac") {
+    if (clang_base_path != "") {
+      ar = rebase_path(clang_base_path, root_build_dir) + "/bin/llvm-ar"
+    } else {
       ar = "ar"
     }
+  }
 
   toolchain_args = {
     current_os = host_os
     current_cpu = host_cpu
   }
 }
 
 # This template defines a toolchain that uses just-built clang and lld