[libc++] Install git-clang-format on CI nodes.

Two problems fixed:
* an old version of clang-format get installed by default (6.0).
* git-clang-format is not present, only git-clang-format-<version> (e.g. git-clang-format-6.0).

Solution:
* install clang-format-11 with explicit version
* make symlink git-clang-format to the latest version of git-clang-format-<version>

Differential Revision: https://reviews.llvm.org/D93201