[orc] Fix unit tests that use ORC C API
* c_api_tests was failing to build after the API change to
__orc_rt_CWrapperFunctionResultAllocate
* wrapper_function_utils_test was causing an assertion failure, because
it was creating a result for `void(void)` with Size = 0, but seeing an
uninitialized pointer, which it considered to be an out-of-bound
error.
I noticed locally that making modifications to c_api.h is not causing
these unit tests to be rebuilt, which may be how the bug slipped in in
the first place.
Differential Revision: https://reviews.llvm.org/D108649