gopls/internal/golang: refine crash report golang/go#69362

This CL splits the various fallible checks that may have
been responsible for the crash report in golang/go#69362 onto
separate lines so that we can confirm whether a nil
map entry is indeed the cause. (I am almost certain that
it is, but I still can't explain it.)

Updates golang/go#69362

Change-Id: I0469e285bda65c21e80a348af04ea0e69f6a31c0
Reviewed-on: https://go-review.googlesource.com/c/tools/+/627015
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: adonovan

gopls/internal/golang/hover.go

@@ -344,6 +344,13 @@ func hover(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle, pp pro
 	// if they embed platform-variant types.
 	//
 	var sizeOffset string // optional size/offset description
+	// debugging #69362: unexpected nil Defs[ident] value (?)
+	_ = ident.Pos()          // (can't be nil due to check after referencedObject)
+	_ = pkg.TypesInfo()      // (can't be nil due to check in call to inferredSignature)
+	_ = pkg.TypesInfo().Defs // (can't be nil due to nature of cache.Package)
+	if def, ok := pkg.TypesInfo().Defs[ident]; ok {
+		_ = def.Pos() // can't be nil due to reasoning in #69362.
+	}
 	if def, ok := pkg.TypesInfo().Defs[ident]; ok && ident.Pos() == def.Pos() {
 		// This is the declaring identifier.
 		// (We can't simply use ident.Pos() == obj.Pos() because