The Acquire method is nearly instantaneous; it only potentially blocks
on a small, constant sequence of cache misses, so there is no need to
avoid blocking in it when a Context is cancelled.

An early return when the passed-in Context is canceled was added in CL
242838 to avoid incrementing the Generation's WaitGroup after its
destruction has begun; however, that early return also bypasses the
WaitGroup accounting that blocks Destroy while the generation is still
in use. Instead, we need the invariant that Acquire is not called in
the first place after Destroy, which we can ensure by nilling out
the View's snapshot when we begin destroying it.

I was not able to reproduce golang/go#48774 locally, but I believe
that this CL will fix it. (It may, however, expose other races or
deadlocks that may have been masked by the early return, which we can
then fix separately.)

Fixes golang/go#48774

Change-Id: Iac36fceb06485f849da5ba0250b44b55f937c44b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/367675
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>