[PERF] remove pointee types from cg_ssa and cg_llvm

[erikdesjardins]

Modern LLVM (15+ by default) doesn't have pointee types, so computing them is a waste of time.

This can't be merged in its current state, because:

• current LLVM 15 allows -Cllvm-args="-opaque-pointers=0" (not sure that we should support this configuration though)
• we support older LLVM versions without fully working opaque pointers

Workarounds could be:

• cx.type_ptr_to(|| ...compute pointee type...), closure only called in LLVM < 15 (kinda awkward)
• targeted if llvm_version < 15 { cx.type_ptr_to(...compute pointee type...) } else { cx.type_i8p() } only at important callsites (might have to end up doing this)

This is just to measure the max possible perf win from everything.

r? @ghost

@rustbot label S-experimental