compiler: support recursive types

Previously, the cycle was broken by inserting an unsafe.Pointer type in
some places. This is of course incorrect, and makes debugging harder.
However, LLVM provides a way to make temporary nodes that are later
replaced, exactly for this purpose.

This commit uses those temporary metadata nodes to allow such recursive
types.

testdata/structs.txt

@@ -9,3 +9,4 @@ test5 1 2 3
 test6 foo 3 5
 test7 (0:nil) 8
 test8 2 3 12 13 6
+test9 nil