[clang-format] Fix preprocessor nesting after commit 529aa4b0

In https://github.com/llvm/llvm-project/commit/529aa4b011c4ae808d658022ef643c44dd9b2c9c
by setting the identifier info to nullptr, we started to subtly
interfere with the parts in the beginning of the function,
https://github.com/llvm/llvm-project/blob/529aa4b011c4ae808d658022ef643c44dd9b2c9c/clang/lib/Format/UnwrappedLineParser.cpp#L991
causing the preprocessor nesting to change in some cases. E.g., for the
added regression test, clang-format started incorrectly guessing the
language as C++.

This tries to address this by introducing an internal identifier info
element to use instead.

Reviewed By: curdeius, MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D120315