Move calling convention conversion into TargetABI.

The code is tightly coupled to TargetABI (the transformations
there only make sense knowing that the right CC is selected).

gen/abi.cpp

@@ -38,6 +38,24 @@ void ABIRewrite::getL(Type* dty, DValue* v, llvm::Value* lval)
 // Some reasonable defaults for when we don't know what ABI to use.
 struct UnknownTargetABI : TargetABI
 {
+    llvm::CallingConv::ID callingConv(LINK l)
+    {
+        switch (l)
+        {
+        case LINKc:
+        case LINKcpp:
+        case LINKintrinsic:
+        case LINKpascal:
+        case LINKwindows:
+            return llvm::CallingConv::C;
+        case LINKd:
+        case LINKdefault:
+            return llvm::CallingConv::Fast;
+        default:
+            llvm_unreachable("Unhandled D linkage type.");
+        }
+    }
+
     bool returnInArg(TypeFunction* tf)
     {
 #if DMDV2
@@ -91,6 +109,11 @@ struct IntrinsicABI : TargetABI
 {
     RemoveStructPadding remove_padding;
 
+    llvm::CallingConv::ID callingConv(LINK l)
+    {
+        return llvm::CallingConv::C;
+    }
+
     bool returnInArg(TypeFunction* tf)
     {
         return false;