LLVM IR code generator refactoring and updates

[grendello]

Context: https://llvm.org/docs/OpaquePointers.html

• Migrate to opaque pointers (LLVM 15+) which will be the only ones
  supported from LLVM 16 onwards. This will allow us to update our
  toolchain to LLVM 16 or newer at some point. It also simplifies
  both the code generator and the generated code.
• Change code generation model. Now the "client" classes don't need
  to concern themselves with how the LLVM IR code looks and how
  it is formatted. Instead, they build a model of the code they want
  to output and let the generator do the rest.
• Handle many more tasks automatically:
  • LLVM IR string management and registration
  • Buffer management for structures which have pointers to buffers
  • References to local variables and strings
  • Local temporary counting (unnamed labels and function parameters).
    LLVM requires that all unnamed local temporaries (that is labels and
    function parameters) are named using a shared counter, with all of thus
    generated names being numbers in sequence. In the previous version of
    the generator it was problematic, in this version it's easy and seamless.