[eudsl-tblgen] add lots of stuff (support retrieving ArgumentInits from Record)

[makslevental]

This is kind of a niche PR so I don't expect anyone to really closely review but I'm describing it anyway just to capture some findings.

The basic feature added in this PR is the ability to recover from an instantiated def which template args were used in the class call/invocation/thing. E.g.,

class Test_Op<string mnemonic, list<Trait> traits = []>
    : Op<Test_Dialect, mnemonic, traits>;

def Test_AndOp : Test_Op<"and"> { ... }

With this PR you can recover that the mnemonic passed as the first argument to Test_Op when instantiating Test_AndOp is "and". This sounds like something that should've been always possible but turns out tablegen literally doesn't keep that information in any way/shape/form. In order to make this work I had to hack the parser to keep it around (note, this isn't completely straightforward because arguments can have unevaluated bang operators).

For context, I intended on using this functionality to translate LLVM Intrinsics into MLIR OpDefs but turns out it didn't really work out (not because the feature doesn't work but because I have ~30k Intrinsics lol).