Commit d813af73f70f addressed a regression introduced by commit
3791b3fca6ea
but caused `clang-format -dump-config` to "hang".
This patch reverts changes to ClangFormat.cpp by both commits and
reworks the cleanup.
Fixes#80621.
The code cleanup in #74794 accidentally broke detection of languages by
reading file content from stdin, e.g. via `clang-format -dump-config - <
/path/to/filename`.
This PR adds unit and integration tests to reproduce the issue and adds
a fix.
Fixes: #79023