Support LLVM Flang

[rouson]

Currently, the command

fpm test --compiler flang-new --flag "-mmlir -allow-assumed-rank"

yields the trailing output

[ 57%]        inference_engine_m_.f90  done.

error: Semantic errors in ././src/inference_engine/inference_engine_m_.f90
./././src/inference_engine/inference_engine_m_.f90:72:32: error: Result of pure function may not have polymorphic ALLOCATABLE ultimate component '%activation_strategy_'
        type(inference_engine_t) inference_engine
                                 ^^^^^^^^^^^^^^^^
./././src/inference_engine/inference_engine_m_.f90:30:50: Declaration of 'activation_strategy_'
      class(activation_strategy_t), allocatable :: activation_strategy_ ! Strategy Pattern facilitates elemental activation
                                                   ^^^^^^^^^^^^^^^^^^^^
./././src/inference_engine/inference_engine_m_.f90:104:24: error: Result of pure function may not have polymorphic ALLOCATABLE ultimate component '%activation_strategy_'
        type(exchange_t) exchange
                         ^^^^^^^^
./././src/inference_engine/inference_engine_m_.f90:52:50: Declaration of 'activation_strategy_'
      class(activation_strategy_t), allocatable :: activation_strategy_ ! Strategy Pattern facilitates elemental activation
                                                   ^^^^^^^^^^^^^^^^^^^^
<ERROR> Compilation failed for object " src_inference_engine_inference_engine_m_.f90.o "
<ERROR> stopping due to failed compilation
STOP 1

[rouson]

Update: Flang-new can now build Inference-Engine and 31 of 33 tests pass. The two failing tests involve training unit tests that don't converge. Those same failing tests pass with the GNU gfortran, Intel ifx, and NAG nagfor compilers.

[rouson]

Release 0.12.0 addresses this.