[clangd] Discard diagnostics from another SourceManager.

This can happen when building implicit modules, as demonstrated in test.
The CompilerInstance uses the same StoredDiags, but different
SourceManager. This used to crash clangd when it tried to relocate the
diagnostic to the main file, which, according to SourceManager from the
diagnostic, is a fake <module-includes> file.

Differential Revision: https://reviews.llvm.org/D85753