[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