[llvm] [offload] Add missing build dependency (PR #149326)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur approved this pull request.

Since in a bootstrapping build the just-built Clang is being used, and offload-tblgen is writen in C++, requiring the libc++ includes to be builts before makes sense (unless Clang decides to use a a libstdc++ installation).

It looks like the entire tblgen setup will not work when building the runtimes for other targets than "default", but that's another issue.

https://github.com/llvm/llvm-project/pull/149326