chore: improve error message for unassigned structs

[tserg]

What I did

Unassigned structs currently run into an error while raising an exception:

struct Foo:
    a: uint256

@external
def foo():
    Foo({a: 1})

AttributeError: 'TYPE_T' object has no attribute '_id'

This is due to TYPE_T not having an _id attribute:

vyper/vyper/semantics/analysis/local.py

Lines 512 to 520 in 8ebabc5

	if (
	return_value
	and not isinstance(fn_type, MemberFunctionT)
	and not isinstance(fn_type, ContractFunctionT)
	):
	raise StructureException(
	f"Function '{fn_type._id}' cannot be called without assigning the result", node
	)
	self.expr_visitor.visit(node.value)

Even if we remove ._id, the error message is still not precise because it is a struct creation and not a function call:

vyper.exceptions.StructureException: Function 'type(Foo declaration object)' cannot be called without assigning the result
  contract "contracts/struct_assign.vy:8", function "foo", line 8:4 
       7 def foo():
  ---> 8     Foo({a: 1})
  -----------^
       9

How I did it

Added a new branch to catch unassigned structs.

How to verify it

See test.

Commit message

chore: improve error message for unassigned structs

Description for the changelog

Improve error message for unassigned structs

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[codecov-commenter]

Codecov Report

Merging #3312 (300259b) into master (8ebabc5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master    #3312   +/-   ##
=======================================
  Coverage   88.89%   88.89%           
=======================================
  Files          84       84           
  Lines       10600    10602    +2     
  Branches     2213     2214    +1     
=======================================
+ Hits         9423     9425    +2     
  Misses        769      769           
  Partials      408      408           

Impacted Files	Coverage Δ
vyper/semantics/analysis/local.py	91.36% <100.00%> (+0.05%)	⬆️

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[tserg]

Note - this now catches builtin functions that have not been folded:

vyper/vyper/semantics/analysis/local.py

Lines 512 to 519 in 8ebabc5

	if (
	return_value
	and not isinstance(fn_type, MemberFunctionT)
	and not isinstance(fn_type, ContractFunctionT)
	):
	raise StructureException(
	f"Function '{fn_type._id}' cannot be called without assigning the result", node
	)