wrangle the llvm bitcode CLI options

[andrewrk]

With #16270 in mind, let's reevaluate the CLI options that relate to LLVM IR modules.

-femit-llvm-bc and -femit-llvm-ir: Currently this is post LLVM optimization. This should continue to be the case, however these options should print an error if you try to use them with a build of zig that does not have LLVM enabled. When #16270 is implemented, these options will disappear entirely. Before that, they should be renamed into -femit-llvm-opt-bc and -femit-llvm-opt-ir.

-ofmt=bc: This option should be introduced, making LLVM IR modules an additional object format that can be selected in a similar fashion to the C backend (-ofmt=c). This is how you specify to emit LLVM bitcode modules with a zig compiler that does not have LLVM enabled. When #16270 is implemented, -fllvm and -fno-llvm disappear entirely but -ofmt=bc remains. When this target is selected, Zig optimization pipeline is run but LLVM's is not.

--verbose-llvm-ir: Should be changed into -femit-llvm-ir for debugging convenience. Corresponding exactly to -ofmt=bc, this emits LLVM IR after Zig optimizations; before LLVM optimizations. After #16270 is implemented, this (changed) option remains because it is useful in combination with -ofmt=bc.