[clangd] Include Cleaner: ignore headers with IWYU export pragmas

Disable the warnings with `IWYU pragma: export` or `begin_exports` +
`end_exports` until we have support for these pragmas. There are too many
false-positive warnings for the headers that have the correct pragmas for now
and it makes the user experience very unpleasant.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D125468