gopls/internal/robustio: only define ERROR_SHARING_VIOLATION on Windows

The type syscall.Errno is not defined on plan9,
so defining a constant of that type in an unconstrained file
causes build errors on the plan9 builders.

The ERROR_SHARING_VIOLATION constant is only needed for Windows and
only makes sense on that platform, so we move it to a Windows-only
file.

Change-Id: I4fbad994a71f746523557f82c72c08ddd5fbcb2e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/454501
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>