[clangd][SwapIndex] ensure that the old index is alive while we are using it...

[clangd][SwapIndex] ensure that the old index is alive while we are using it via the function returned by `SwapIndex::indexedFiles()` call

Without this patch the old index could be freed, but there still could be tries to access it via the function returned by `SwapIndex::indexedFiles()` call.
This leads to hard to reproduce clangd crashes at code completion.
This patch keeps the old index alive until the function returned by `SwapIndex::indexedFiles()` call is alive.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D95206