Handle dynamic constant assignment

[amomchilov]

Motivation

Closes #293 by reimplementing this workaround done by Sorbet's parser:

sorbet/parser/Builder.cc

Lines 348 to 354 in 97d7fff

	} else if (auto *c = parser::cast_node<Const>(node.get())) {
	if (driver_->lex.context.inDef) {
	error(ruby_parser::dclass::DynamicConst, node->loc);
	// Error recovery: lie and say it was an assign to something else so that the parse can continue
	auto name = core::Names::dynamicConstAssign();
	driver_->lex.declare(name.shortName(gs_));
	return make_unique<LVarLhs>(c->loc, name);

[amomchilov]

I could have extracted this up out of the conditional, like this:

auto prismName = parser.resolveConstant(node->name);
auto sorbetName = gs.enterNameConstant(name);

if (isInMethodDef) {
  return make_unique<LVarLhs>(location, core::Names::dynamicConstAssign());
}

return make_unique<SorbetLHSNode>(location, move(parent), sorbetName);

... but I opted not to, because it makes it look like sorbetName is an unused variable in the early-return. Repeating it here gives me a place to explicit document the side-effect of enterNameConstant that we're relying on.

[amomchilov]

Hehe this was never the default constructor, since it had parameters. Such are the perils of comments.

[amomchilov]

I had to be less specific about the return type, to account for the case where if SorbetLHSNode is parser::CVarLHS, where we need to return a parser::LVarLhs for the workaround instead.

[egiurleo]

Woohoo! Looks like you might need a rebase.