[clangd] Suppress IncludeCleaner warnings for headers behind pragma keep

D114072 allows filtering out the warnings for headers behind `// IWYU pragma:
keep`. This is the first step towards more useful IWYU pragmas support and
fine-grained control over the IncludeCleaner warnings.

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D115345