Implement another D-specific pass: -dgc2stack This one promotes GC allocations to stack memory when it can determine it's safe to do so. Not all GC calls are recognized yet (in fact only one *is* recognized for now). Needs metadata, so disabled for LLVM versions that don't support it.

enum
{
    LLVMnone,
    LLVMintrinsic,
    LLVMno_typeinfo,
    LLVMno_moduleinfo,
    LLVMalloca,
    LLVMva_start,
    LLVMva_copy,
    LLVMva_end,
    LLVMva_arg,
    LLVMldc,
    LLVMinline_asm
};