As clang-format 3.9 is not available on Focal, I would propose to set the recommended version of clang-format to 8 which is available on Xenial, Bionic and Focal.
See also: https://packages.ubuntu.com/search?searchon=names&suite=all§ion=all&keywords=clang-format
(Unfortunately, the same .clang-format file leads to different results when used on different versions of clang-format.)
As clang-format 3.9 is not available on Focal, I would propose to set the recommended version of clang-format to 8 which is available on Xenial, Bionic and Focal.
See also: https://packages.ubuntu.com/search?searchon=names&suite=all§ion=all&keywords=clang-format
(Unfortunately, the same
.clang-formatfile leads to different results when used on different versions of clang-format.)