Implement codegen expression `Expression::StringLiteral`

[xermicus]

          As we don't use `AllocDynamicBytes`, we could just have an `Expression::StringLiteral` to cover this case.

Originally posted by @LucasSte in https://github.com/hyperledger/solang/pull/1449#discussion_r1269462467

This would be useful in a number of places, including the linked PR.

In emit, it should just yield a interned global string, the same as Expression::AllocDynamicBytes does if the Type is Slice