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.

gen/passes/Passes.h

@@ -7,6 +7,7 @@ namespace llvm {
 
 // Performs simplifications on runtime calls.
 llvm::FunctionPass* createSimplifyDRuntimeCalls();
+llvm::FunctionPass* createGarbageCollect2Stack();
 
 
 #endif