x/tools/gopls: crash in signature help

[adonovan]

This bug.Reportf stack was was reported by telemetry:

golang.org/x/tools/gopls/internal/bug.report:35
golang.org/x/tools/gopls/internal/bug.Errorf:2
golang.org/x/tools/gopls/internal/lsp/source.SignatureHelp:72
golang.org/x/tools/gopls/internal/lsp.(*Server).signatureHelp:9
golang.org/x/tools/gopls/internal/lsp.(*Server).SignatureHelp:1
golang.org/x/tools/gopls/internal/lsp/protocol.serverDispatch:490
golang.org/x/tools/gopls/internal/lsp/lsprpc.(*StreamServer).ServeStream.ServerHandler.func3:5
golang.org/x/tools/gopls/internal/lsp/lsprpc.(*StreamServer).ServeStream.handshaker.func4:52
golang.org/x/tools/gopls/internal/lsp/protocol.Handlers.MustReplyHandler.func1:2
golang.org/x/tools/gopls/internal/lsp/protocol.Handlers.AsyncHandler.func2.2:3

It looks like I forgot to handle the case of a conversion T(x) where T is a built-in type:

	// Get the object representing the function, if available.
	// There is no object in certain cases such as calling a function returned by
	// a function (e.g. "foo()()").
	var obj types.Object
	switch t := callExpr.Fun.(type) {
	case *ast.Ident:
		obj = pkg.GetTypesInfo().ObjectOf(t)
	case *ast.SelectorExpr:
		obj = pkg.GetTypesInfo().ObjectOf(t.Sel)
	}

	// Built-in?
	if obj != nil && !obj.Pos().IsValid() {
		// built-in function?
		if obj, ok := obj.(*types.Builtin); ok {
			return builtinSignature(ctx, snapshot, callExpr, obj.Name(), pos)
		}

		// error.Error?
		if fn, ok := obj.(*types.Func); ok && fn.Name() == "Error" {
			return &protocol.SignatureInformation{
				Label:         "Error()",
				Documentation: stringToSigInfoDocumentation("Error returns the error message.", snapshot.Options()),
			}, 0, nil
		}
		// [MISSING LOGIC for T(x) conversion here]
		return nil, 0, bug.Errorf("call to unexpected built-in %v (%T)", obj, obj)
	}

[gopherbot]

Change https://go.dev/cl/542057 mentions this issue: gopls/internal/lsp/source: fix panic on T(x) conversion