interp: add support for constant type asserts

Non-constant type asserts are not yet implemented, but should be
relatively easy to add at a later time. They should result in a clear
error message for now.

interp/testdata/interface.out.ll

@@ -0,0 +1,9 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64--linux"
+
+@main.v1 = local_unnamed_addr global i1 true
+
+define void @runtime.initAll() unnamed_addr {
+entry:
+  ret void
+}