Compiling C tests with C++ does not detect all problems

There is some C++ code in our C headers at ``sidre/core/SidreDataTypeIds.h.

using SIDRE_IndexType = int64_t;

Since we're compiling the C tests with C++, everything seemed ok.
I made a simple version of ``sidre_group_C.cpp`` as ``sidre_group_C_c.c`` with the addition of some lines

#include
#include
#define TEST(a, b) void a##_##b(void)
#define EXPECT_TRUE(a) assert(a)
#define EXPECT_FALSE(a) assert(!(a))
#define EXPECT_EQ(a, b) assert(a == b)

And a simple main program:

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

Produces the error:

../../axom/sidre/core/SidreDataTypeIds.h:22:1: error: unknown type name 'using'; did you mean 'uint'?
using SIDRE_IndexType = int64_t;
^~~~~

I would find using the C compiler useful when testing Shroud changes to keep us honest.

There are a plethora of unit testing frameworks:
https://en.wikipedia.org/wiki/List_of_unit_testing_frameworks#C

Start with sidre_group_C.cpp, the proposed sidre_group_C_c.c, and the C header sidre/core/SidreDataTypeIds.h. Review how the existing C tests are built, then run the tests with a C compiler; done means the C test sources compile and run independently of C++ and expose C-incompatible headers such as the shown using declaration.

Written by the indexing model from the issue text.