Running syntax tests via IR

## Abstract

Currently our syntax tests get compiled only using the legacy codegen. We should either switch to compiling via IR or compile using both.

## Motivation

Compiling via IR would increase test coverage and and bugs like #15669 show that we really need that.

This was also originally an issue for EOF implementation because it only works via IR.

Note that this is likely going to significantly increase the test running time.

## Specification

We already have `compileViaYul` flag in syntax tests: https://github.com/ethereum/solidity/blob/13943d9c24c805dc4df9416fda190becd163976e/test/libsolidity/SyntaxTest.cpp#L51

The solution would be to either switch the default to `true` or start supporting `also` like in semantic tests to compile both ways (and make it the default). Then fix any issues that come up in the tests.