1201: Add -Werror for CI builds that are warning clean

[cz4rs]

fixes #1201

[cz4rs]

I was overly optimistic with this because I was using Debug configuration in local (which passes happily)

I will take a quick look at how much work it would take to make basic gcc and clang builds to pass

[PhilMiller]

There's a funny quirk of compiler diagnostics, that some of them only become apparent with optimizations enabled, because the analysis that makes the need for a diagnostic apparent is part of an optimization pass. Those passes are skipped with optimizations disabled for the sake of performance.

[codecov]

Codecov Report

Merging #1209 (ce570a7) into develop (e87c3f9) will increase coverage by 0.00%.
The diff coverage is 86.66%.

@@           Coverage Diff            @@
##           develop    #1209   +/-   ##
========================================
  Coverage    80.99%   81.00%           
========================================
  Files          730      730           
  Lines        28046    28044    -2     
========================================
  Hits         22716    22716           
+ Misses        5330     5328    -2     

Impacted Files	Coverage Δ
src/vt/messaging/active.cc	84.30% <ø> (ø)
src/vt/runtime/component/diagnostic_value.h	88.34% <ø> (ø)
...unit/sequencer/test_sequencer_parallel.extended.cc	100.00% <ø> (ø)
.../vt/vrt/collection/balance/stats_restart_reader.cc	82.55% <81.81%> (ø)
tests/unit/collection/test_lbstats_retention.cc	100.00% <100.00%> (ø)
tests/unit/diagnostics/test_diagnostic_value.cc	95.65% <100.00%> (ø)
src/vt/runtime/runtime_diagnostics.cc	0.00% <0.00%> (ø)
src/vt/configs/error/pretty_print_message.cc	3.33% <0.00%> (+0.10%)	⬆️

[cz4rs]

note to self:

• update documentation (build section) with new configuration variables
• make sure that builds fail as expected when a warning is introduced

[cz4rs]

I have fixed a couple of simple warnings which allowed to enable -Werror for all builds except:

• nvcc builds (it will not fail even when warnings are present)
• extended builds
• gcc-5 - raises defined but not used for the code which uses #pragma GCC diagnostic ignored "-Wunused-variable"
• gcc-8 - asan build, complains about vt::runtime::component::detail::DiagnosticValueWrapper members being potentially used uninitialized ([-Werror=maybe-uninitialized])

[cz4rs]

note: this is disabled by default, enabled only for selected builds - up for debate if we should turn this around

[lifflander]

• DiagnosticValueWrapper

For DiagnosticValueWrapper, we should change this line T initial_value_; to T initial_value_ = {};

[lifflander]

Overall, looks good. I think the default compiler should probably be changed back unless there was a reason.

[cz4rs]

• -Werror enabled for all gcc and clang builds except for gcc-5 (which does not respect pragma GCC diagnostic ignored "-Wunused-variable" we have in the code, seems like it starts to work with GCC >= 6)
• regarding DiagnosticValueWrapper, it might be better to simply remove default ctor for this class (requires Unpacking of containers default-constructs elements rather than using reconstruction logic magistrate#161)
• handle errors correctly when reading files in StatsRestartReader #1213 created to handle error checking in StatsRestartReader properly

[jstrzebonski]

Looks good to me