Always insert global symbols to _global_symbols module

[czgdp1807]

Closes #1727
#1727 (comment)

[Thirumalai-Shaktivel]

LGTM! Thanks for fixing this. This was on my TODO list, but I kept forgetting about it. 
So, from now on, we will always have two symbols in the TranslationUnit. If the program uses an IntrinsicFunction then the TranslationUnit also includes that function in the global scope.

Empty program:

(TranslationUnit
    (SymbolTable
        1
        {
            _global_symbols:
                (Module
                    (SymbolTable
                        3
                        {
                            
                        })
                    _global_symbols
                    []
                    .false.
                    .false.
                ),
            main_program:
                (Program
                    (SymbolTable
                        2
                        {
                            
                        })
                    main_program
                    []
                    []
                )
        })
    []
)

[czgdp1807]

Thanks for pointing out. I don't create _global_symbols now if global scope doesn't have any user defined symbols. Now the behaviour will be same as main branch when the input file is empty.

[certik]

I think it looks good, thanks for fixing it!

[certik]

Hm, this PR broke --disable-main, it now always inserts main:

(lp) lpython/integration_tests(main) $ lpython --show-c --disable-main import_order_01.py -I.
#include <inttypes.h>

#include <stdlib.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <lfortran_intrinsics.h>

#define ASSERT(cond)                                                           \
    {                                                                          \
        if (!(cond)) {                                                         \
            printf("%s%s", "ASSERT failed: ", __FILE__);                       \
            printf("%s%s", "\nfunction ", __func__);                           \
            printf("%s%d%s", "(), line number ", __LINE__, " at \n");          \
            printf("%s%s", #cond, "\n");                                       \
            exit(1);                                                           \
        }                                                                      \
    }
#define ASSERT_MSG(cond, msg)                                                  \
    {                                                                          \
        if (!(cond)) {                                                         \
            printf("%s%s", "ASSERT failed: ", __FILE__);                       \
            printf("%s%s", "\nfunction ", __func__);                           \
            printf("%s%d%s", "(), line number ", __LINE__, " at \n");          \
            printf("%s%s", #cond, "\n");                                       \
            printf("%s", "ERROR MESSAGE:\n");                                  \
            printf("%s%s", msg, "\n");                                         \
            exit(1);                                                           \
        }                                                                      \
    }


struct dimension_descriptor
{
    int32_t lower_bound, length;
};

// Implementations
int32_t f()
{
    int32_t _lpython_return_variable;
    _lpython_return_variable = 42;
    return _lpython_return_variable;
}

void main1()
{
    int32_t a;
    a = f();
    printf("%d\n", a);
}

int main(int argc, char* argv[])
{
    return 0;
}

Hopefully it can be fixed with a subsequent PR, otherwise we'll need to revert it.

[czgdp1807]

Let me see. Probably a corner case.