[Refactor] Reference argument usage in C++

[seanlatias]

We should enforce ourselves to do the following (according to Google C++ style):

void (const A& a); //good
void (A* a); //good
void (A& a); //bad

This is also a common coding style (at least TVM is also requiring this). We should create a PR purely for cleaning up this usage. Tagging the current contributors.

@Hecmay @paldebjit @zzzDavid @Sibylau

[seanlatias]

This applies not only to function arguments but also class constructors.

[chhzh123]

As we have released a new version of HeteroCL that entirely moves the compilation stack to MLIR, issues related to the original HalideIR flow will be closed.