Change value witnesses to take the metatype as 'self'.

The motivations here are that (1) the parametric types
that actually need the 'self' argument don't necessarily
all want to do what tuples do and put the VWT relative
to the metatype at some definable offset and (2)
recovering type parameters from the metatype is much
better defined than also hopping some relationship back.
Plus this allows VWTs to be shared across instances of
generic types.  Also, I'm going to need to add a VW
that takes a metatype, and consistency seems right here.

If keeping two values live is actually punishing, I
might have to reconsider this.  But the VWT is at least
always recoverable from the metatype, so....

I ended up abstracting the thing that GenHeap was doing
in order to save archetypes for arrays, because I
needed it to save metatypes instead of VWTs and because
it really needed abstractin'.

Swift SVN r3096

lib/IRGen/StructLayout.cpp

@@ -45,11 +45,17 @@ static bool requiresHeapHeader(LayoutKind kind) {
   llvm_unreachable("bad layout kind!");
 }
 
-void swift::irgen::addHeapHeaderToLayout(IRGenModule &IGM,
-                                         Size &size, Alignment &align,
-                                         SmallVectorImpl<llvm::Type*> &fields) {
+/// Return the size of the standard heap header.
+Size irgen::getHeapHeaderSize(IRGenModule &IGM) {
+  return IGM.getPointerSize() * 2;
+}
+
+/// Add the fields for the standard heap header to the given layout.
+void irgen::addHeapHeaderToLayout(IRGenModule &IGM,
+                                  Size &size, Alignment &align,
+                                  SmallVectorImpl<llvm::Type*> &fields) {
   assert(size.isZero() && align.isOne() && fields.empty());
-  size += IGM.getPointerSize() * 2;
+  size = getHeapHeaderSize(IGM);
   align = IGM.getPointerAlignment();
   fields.push_back(IGM.RefCountedStructTy);
 }