Teach `-dgc2stack` to promote GC allocations in simple loops to stack allocations too. (A "simple" loop is one where the allocation isn't used in a subsequent iteration) This also means it's no longer necessary to run this pass multiple times. Running it once after inlining should now catch all cases.

// D runtime support helpers

bool LLVM_D_InitRuntime();
void LLVM_D_FreeRuntime();

llvm::Function* LLVM_D_GetRuntimeFunction(llvm::Module* target, const char* name);

llvm::GlobalVariable* LLVM_D_GetRuntimeGlobal(llvm::Module* target, const char* name);