Unify creating clang source tarball creation into one project
While working on #40014 I realized that a bunch of projects (clang,
fenix-clang, mingw-w64-clang, and macosx-toolchain +
wasi-sysroot in #40014) do the dance of assembling a clang source
tarball (fully or partly) and then start using that one. We should
abstract that into an own project.
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