[clangd] Adjust diagnostic range to be inside main file

Summary:
LSP requires diagnostics to lay inside main file. In clangd we keep
diagnostics in three different cases:
- already in main file
- adjusted to a header included in main file
- has a note covering some range in main file

In the last case, we were not adjusting the diagnostics range to be in main
file, therefore these diagnostics ended up pointing some arbitrary locations.

This patch fixes that issue by adjusting the range of diagnostics to be the
first note inside main file when converting to LSP.

Reviewers: ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72458