Set the language version from the controlling go.mod file's go version,
if any. Also verify that we properly surface a diagnostic if the version
is invalid.
I didn't add any quick fixes.
Fixesgolang/go#50688.
Change-Id: Ic472502d1224a1decb5b989d51110b837020e998
Reviewed-on: https://go-review.googlesource.com/c/tools/+/383394
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>