internal/types: rename 54942.go to issue54942.go

This filename was inconsistent with all others.

Change-Id: I5abd3792a96c5137f45d02ac51ffe3d8d05bb164
Reviewed-on: https://go-review.googlesource.com/c/go/+/484415
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>

diff --git a/src/internal/types/testdata/fixedbugs/54942.go b/src/internal/types/testdata/fixedbugs/issue54942.go
similarity index 100%
rename from src/internal/types/testdata/fixedbugs/54942.go
rename to src/internal/types/testdata/fixedbugs/issue54942.go