New repository clang/llvm-project

Please set up a new repository

https://codereview.qt-project.org/admin/repos/clang/llvm-project

that is synced from

https://github.com/llvm/llvm-project.git

in the same manner as the already existing clang/* projects, e.g.
clang/clang. Please also grant the same permissions for the new
repository as for the others. That is, pushing tags should be possible.

Background: https://github.com/llvm/llvm-project.git is the new official
repository of LLVM [1]. It combines the former repositories we are
syncing right now (clang/*, except clazy as it is not part off llvm)
into a single one. Since this "mono repository" is much easier to deal
with and I'm not sure for how long the others will stay around, I would
like to follow suit so that we can get rid of the current

clang/llvm
clang/clang
clang/clang-tools-extra

in the long run.