Add saw-script llvm_struct_type function

[brianhuffman]

We have a function llvm_struct_value : [SetupValue] -> SetupValue for creating LLVM values in saw-script, but we do not have a corresponding function llvm_struct_type : [LLVMType] -> LLVMType for creating LLVM struct types. We should add it.

Note that we do already have a function called llvm_struct that gives you an LLVMType, but it only takes a string as an argument and gives you the corresponding alias type (which is often, but not necessarily, defined as a struct type).

The llvm_struct_type function will be specifically useful when you have a program that has defined a variable-length struct (i.e. one where the last field is an array with unspecified length, which is compiled to LLVM with an array length of 0) and you want to allocate one with a non-zero array length in the last field.

For completeness, we should probably also add a variant for packed struct types too, although these are rarely used in practice.

[RyanGlScott]

Note that we do already have a function called llvm_struct that gives you an LLVMType, but it only takes a string as an argument and gives you the corresponding alias type (which is often, but not necessarily, defined as a struct type).

Wow, that's pretty confusing. I wonder if we should use a name like llvm_alias instead for this?

[brianhuffman]

One problem with renaming llvm_struct to llvm_alias is that llvm_struct is already in use in a lot of places, so renaming it would break a bunch of existing proof scripts.

I suppose we could always just introduce a new name llvm_alias, and then phase out the old name later.

[RyanGlScott]

I suppose we could always just introduce a new name llvm_alias, and then phase out the old name later.

Right, this is what I was thinking. Having llvm_struct coexist with llvm_struct_type seems ripe for confusion.