[clangd] Record IWYU pragma keep in the IncludeStructure

[clangd] Record IWYU pragma keep in the IncludeStructure

This will allow the IncludeCleaner to suppress warnings on the lines with "IWYU
pragma: keep".

Clang APIs are not very convinient, so the code has to navigate around it.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D114072