[clangd] Retain main file fixes attached to diags from preamble

Clangd ignores fixits if the diagnsotics is outside the main file (e.g.
a note on a declaration from a header), but the fix might still be inside the
main file (e.g. change the function call).

This patch changes the logic to retain fixes that touch main file, if the
diagnostic owning them is still inside main file, even if they are attached to a
note outside the main file.

Differential Revision: https://reviews.llvm.org/D122315