internal/completion: default to regular completion for f.Fuzz without f.Add
internal/completion: default to regular completion for f.Fuzz without f.Add

Change-Id: I7a05dba3f4a68c2075ebdf078e9e0ee6cfd97f28
Reviewed-on: https://go-review.googlesource.com/c/tools/+/393019
Run-TryBot: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Trust: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools