[clangd] Fix readability-else-after-return 'Adding a note without main diagnostic' crash

Fix a crash in clangd caused by an (admittidly incorrect) Remark diagnositic being emitted from readability-else-after-return.
This crash doesn't occur in clang-tidy so there are no tests there for this.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D81785