Implement RETURN_VALUE_DISCARDED warning in GDscript

[clayjohn]

Fixes: #66100

RETURN_VALUE_DISCARDED wasn't implemented yet in master so no errors were printed when users called functions and ignored the return value

[akien-mga]

It seems to fail the GDScript test suite:

>> The function 'call()' returns a value, but this value is never used.
foo
foo
bar
true


modules/gdscript/tests/gdscript_test_runner.cpp:198: ERROR: CHECK( result.passed ) is NOT correct!
  values: CHECK( false )
  logged: /home/runner/work/godot/godot/modules/gdscript/tests/scripts/runtime/features/params_default_values.gd
          GDTEST_OK
>> WARNING
>> Line: 4
>> RETURN_VALUE_DISCARDED
>> The function 'const_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 5
>> RETURN_VALUE_DISCARDED
>> The function 'func_result_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 9
>> RETURN_VALUE_DISCARDED
>> The function 'func_result_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 10
>> RETURN_VALUE_DISCARDED
>> The function 'lots_of_defaults()' returns a value, but this value is never used.
>> WARNING
>> Line: 12
>> RETURN_VALUE_DISCARDED
>> The function 'ref_default()' returns a value, but this value is never used.
42
1
2
non-optional 1 2 3
non-optional 42 non-optional 41


./modules/gdscript/tests/gdscript_test_runner_suite.h:47: FATAL ERROR: REQUIRE( fail_count == 0 ) is NOT correct!
  values: REQUIRE( 22 == 0 )
  logged: Make sure `*.out` files have expected results.
          All GDScript tests should pass.

[clayjohn]

It seems to fail the GDScript test suite:

>> The function 'call()' returns a value, but this value is never used.
foo
foo
bar
true


modules/gdscript/tests/gdscript_test_runner.cpp:198: ERROR: CHECK( result.passed ) is NOT correct!
  values: CHECK( false )
  logged: /home/runner/work/godot/godot/modules/gdscript/tests/scripts/runtime/features/params_default_values.gd
          GDTEST_OK
>> WARNING
>> Line: 4
>> RETURN_VALUE_DISCARDED
>> The function 'const_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 5
>> RETURN_VALUE_DISCARDED
>> The function 'func_result_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 9
>> RETURN_VALUE_DISCARDED
>> The function 'func_result_default()' returns a value, but this value is never used.
>> WARNING
>> Line: 10
>> RETURN_VALUE_DISCARDED
>> The function 'lots_of_defaults()' returns a value, but this value is never used.
>> WARNING
>> Line: 12
>> RETURN_VALUE_DISCARDED
>> The function 'ref_default()' returns a value, but this value is never used.
42
1
2
non-optional 1 2 3
non-optional 42 non-optional 41


./modules/gdscript/tests/gdscript_test_runner_suite.h:47: FATAL ERROR: REQUIRE( fail_count == 0 ) is NOT correct!
  values: REQUIRE( 22 == 0 )
  logged: Make sure `*.out` files have expected results.
          All GDScript tests should pass.

Looks like it is working! I'll clean up the tests I spoke too soon, these tests shouldn't fail

[clayjohn]

@akien-mga This should be ready for review now

[akien-mga]

Looks good to me.

[akien-mga]

Thanks!