Fix compiler warning for Elixir versions prior to 1.12

[britto]

That warning no longer shows up in Elixir 1.12, but still does in 1.11 and lower. Adding generated: true in a few places fixes it.

warning: the result of the expression is ignored (suppress the warning by assigning the expression to the _ variable)
  lib/protobuf/decoder.ex:103

[britto]

Oddly enough, the warnings are gone in 1.10 and 1.11 but still show up in 1.8 and 1.9 🤔

[ericmj]

Can you add a comment to the file explaining why the generated: true were added and that they can be removed when we only support 1.12+?

If possible please see if you can narrow down which quotes cause the warnings on 1.8 and 1.9 and add the generated: true for them in separate commit. This will help us investigate the underlying issue in the Elixir compiler. Even though it seems to be fixed in 1.12 we don't know why and there's probably no test for it so we can easily have a regression.

[britto]

@ericmj that's a good idea! I will install 1.8 and 1.9 locally tomorrow and try to further narrow it down 🔍

[britto]

@ericmj here's a breakdown of my findings per version:

• 1.12: no warnings, no changes required.
• 1.11 and 1.10 behave the same: this is the minimal set of generated: true required to silence the warnings.
• 1.9 and 1.8 also behave the same: unable to fix even when adding generated: true to all quote calls.