Add a tentative .clang-format for LilyPond. (issue 561340043 by hanwenn@gmail.com)

[lemzwerg--- via Discussions on LilyPond development]

For which clang-format version is this format file?  It doesn't work
with 7.0.1, for example.  Please add this information to the file.

https://codereview.appspot.com/561340043/