When functions are passed as arguments to a lambda, it's possible for
the mangled names of these functions to be the same despite the prototypes
being different. For example:

  int non_streaming_fn(int);
  int streaming_fn(int) __arm_streaming;

  auto lambda_fn = [](const auto &f) { return f(); };
  return lambda_fn(non_streaming_fn) + lambda_fn(streaming_fn);

Only one function will be generated for the lambda above and the __arm_streaming
attribute from streaming_fn will be incorrectly applied to non_streaming_fn.
With this change, an error will be emitted when a duplicate mangled name is
found but the function prototypes do not match.

- Added a test using calling convention attributes

…angling-conflicts.cpp

- Replaced ifdefs in test with use of split-file