x/tools/gopls: memory leak in parsing

[findleyr]

It looks like following recent changes to snapshot accounting, snapshot.ParseGo does not track parse keys modes correctly, causing gopls to hold onto parse results from earlier versions of a file.

This can lead to a significant memory leak over time, particularly for large files.

Fix incoming.

CC @adonovan

[gopherbot]

Change https://go.dev/cl/456642 mentions this issue: gopls/internal/lsp/cache: record parse keys when they're created

[gopherbot]

Change https://go.dev/cl/456815 mentions this issue: [gopls-release-branch.0.11] gopls/internal/lsp/cache: record parse keys when they're created