[libc++] Add InsertNewlineAtEOF=True to .clang-format (#214078)

Newline at the end of a file is a standard convention in the `libc++`
codebase, but it's not enforced via `clang-format`.
Current PR enables this check:

https://clang.llvm.org/docs/ClangFormatStyleOptions.html#insertnewlineateof

GitOrigin-RevId: a5b9b6900655feea616ee2337920b85709d9757d