gopls/internal/golang: downgrade bug.Reportf for missing builtin

I am able to reproduce the bug by commenting out declarations
in builtin.go or unsafe.go, which is very unlikely to happen
in practice but reason enough not to call bug.Reportf.

Fixes golang/go#64604

Change-Id: Idc7e0b2bda0b6070f3d09ba9cb825b7f28b8f796
Reviewed-on: https://go-review.googlesource.com/c/tools/+/560897
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/golang/definition.go

@@ -118,7 +118,9 @@ func builtinDecl(ctx context.Context, snapshot *cache.Snapshot, obj types.Object
 		astObj := file.Scope.Lookup(name)
 		if astObj == nil {
 			// Every built-in should have documentation syntax.
-			return nil, bug.Errorf("internal error: no object for %s", name)
+			// However, it is possible to reach this statement by
+			// commenting out declarations in {builtin,unsafe}.go.
+			return nil, fmt.Errorf("internal error: no object for %s", name)
 		}
 		decl, ok := astObj.Decl.(ast.Node)
 		if !ok {