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