[clang-format] Add the KeepEmptyLinesAtEOF option

Adds an option KeepEmptyLinesAtEOF to keep empty lines (up to
MaxEmptyLinesToKeep) before EOF. This remedies the probably unintentional
change in behavior introduced in 3d3ea84a, which started to always
remove empty lines before EOF.

Fixes #56054.
Fixes #63150.

Differential Revision: https://reviews.llvm.org/D152305