Bug#939825: More info

[Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>]

retitle 939825 ClangFormat: requires patched clang
thanks

Hi! It turns out that the ClangFormat plugin requires a patched Clang. The patch
has been published to upstream but it has so far not been yet accepted.

We will not ask Clang maintainers to push a non-accepted patch. If you the
reader want this to happen please help pushing the patch into clang's main
branch: https://reviews.llvm.org/D53072

In the meantime I pushed a patch into Qt Creator's dev tree in order to give the
user a more detailed error message, I have also just pushed this patch to
unstable.

Cheers, Lisandro.