[FXML-4791] Add printout of references with emitc.reference attr

[cferry-AMD]

So, here's an attempt to have some form of references with EmitC. This PR merely consists of:

• an attribute emitc.reference that, when borne by function arguments or cast operations, results in a reference variable being created,
• some printout code for these references.

To make things smoother for the arrival of a reference type (if that's what upstream would like to see), the printout of types is split into two functions: printReferenceToType, and printType (the existing function). For now, printReferenceToType calls printType but not the other way around, so only the outermost type can bear references (there's no way to make an array of refs for instance); a true reference type would allow nesting and therefore back-and-forth calls between printReferenceToType and printType.

[cferry-AMD]

One thing that seems to be common is to define named attributes of ops in the ODS. Should we also do this for the reference attribute?

[cferry-AMD]

I think we'll have to live with the {emitc.reference} attribute of arguments, as the function op parser doesn't give any flexibility (we can't have a dialect-specific function argument parser). Additionally, it seems highly unwise to make a change impacting all other dialects - Shape, Func, etc - to accomodate a special use of EmitC, especially if a true reference type can work without these changes...

[mgehre-amd]

Looks good!

[cferry-AMD]

I ended up duplicating most of the code in mlir/lib/Interfaces/FunctionImplementation.cpp into mlir/lib/Dialect/EmitC/IR/FunctionOpAssembly.cpp. Now we get a nice printout of function signatures:

emitc.func @f(%x: i32 ref)

[cferry-AMD]

Something tells me upstream won't like the duplicated code very much. But they might be fine with amending the function interfaces that will set custom parsers for arguments and likewise results (in which case we wouldn't have to duplicate so much code, only the code that does the argument list parsing).