fix[lang]: reject type names passed as builtin varargs

[beardthelion]

What I did

Builtins that take variable arguments never semantically checked them, so a bare type name passed as a vararg reached codegen and crashed the compiler with an internal panic. This rejects that case during semantic analysis. Part of #4609

How I did it

BuiltinFunctionT._validate_arg_types inferred each vararg's type only for its side effects and discarded the result. A bare type name like uint256 infers to TYPE_T, which is exactly inferable, so it passed the check and slipped through to codegen, where TYPE_T has no abi_type/typ/location and the compiler panicked. The vararg loop now inspects the inferred type and rejects TYPE_T with InvalidReference, the same "not a variable or literal" error the language already raises for a type name in value position (x: uint256 = uint256).

How to verify it

On master both of these panic:

@external
def foo():
    x: Bytes[32] = abi_encode(uint256)   # CompilerPanic: TYPE_T does not implement abi_type

@external
def bar():
    print(uint256)                       # CodegenPanic: 'IntegerT' object has no attribute 'typ'

After this change both raise InvalidReference: not a variable or literal: 'uint256' at compile time. Regression tests are in tests/functional/syntax/test_abi_encode.py and tests/functional/syntax/test_print.py.

Commit message

Builtins with variable arguments (abi_encode, print, raw_create,
create_from_blueprint) inferred each vararg's type for its side effects
but never checked the argument was a runtime value. A bare type name
infers to TYPE_T, which passed the vararg check and slipped through to
codegen, where it panicked (`TYPE_T does not implement abi_type`,
`'IntegerT' object has no attribute 'typ'`).

Reject a TYPE_T vararg during semantic analysis with the same "not a
variable or literal" error the language already raises for a type name
in value position (e.g. `x: uint256 = uint256`).

Part of GH 4609

Description for the changelog

Reject a bare type name passed as a builtin vararg (e.g. abi_encode(uint256)) with a clear error instead of an internal compiler panic.

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[Sporarum]

Looks very good !

One small potential nitpick below