Consider introducing debug messages in the compiler that are guarded by a flag

[mohammadfawaz]

Inspired by Clang/LLVM, it would be nice to have messages in the compiler that are guarded by a flag that tells us what the compiler is doing at any given point. These could be per compiler stage or optimization pass. The way this works in LLVM is something like:

LLVM_DEBUG(dbg() << "Starting some complex step");

LLVM_DEBUG is just a C++ macro and dbg() is just like println.

These message are hidden unless explicitly asked for in the command line by passing a flag --debug or --debug-only=<stage> or something like that.

It is also important to somehow remove these in production so they're not part of the final compiler binaries. I haven't look into how to do any of that in Rust yet.

As the compiler becomes more complex and the codebase grows larger, we're going to have to debug many complex issues (like #2240) and it would be nice to be able to pass a --debug/--debug-only flag to quickly figure out what stage is problematic before needing an actual debugger.