x/tools/gopls: experimentalDiagnosticsDelay causes diagnostics to flash

[stamblerre]

When a package has only analysis diagnostics and experimentalDiagnosticsDelay is set, the diagnostic cache does not work correctly and diagnostics flash between empty and the results with analysis.

/cc @findleyr

[gopherbot]

Change https://golang.org/cl/267817 mentions this issue: internal/lsp: fix flickering analysis diagnostics