It is already possible to use files fetched by tooltool from a regular build (see https://tbpl.mozilla.org/?tree=Try&rev=fca5d53846e3), but one has to find them by looking at $PWD and different configurations have different relative paths. Most builds use $PWD/../, but universal os X ones need to use $PWD/../../.

Can you try using $topsrcdir/clang instead of $PWD/../clang and $PWD/../../clang?

I pushed https://tbpl.mozilla.org/?tree=Try&rev=d095989da2fd

I stopped the build after verifying that compiler step passes configire and clang compiled a bunch of files. /me declares $topsrcdir working :)