ir: add docs to NewTypeDef to clarify that typ.SetName is invoked

Fixes #226.

ir/module_type.go

@@ -6,6 +6,13 @@ import "github.com/llir/llvm/ir/types"
 
 // NewTypeDef appends a new type definition to the module based on the given
 // type name and underlying type.
+//
+// Note, the name of the given type is set by invoking typ.SetName. As such,
+// users are advised to refrain from creating type definitions from the
+// convenience global variables of the types package (e.g. types.I64), since
+// doing so would change the LLVM IR name of all uses of the types.I64 global
+// variable. Instead, create a new type (e.g. types.NewInt(64)) for this
+// purpose.
 func (m *Module) NewTypeDef(name string, typ types.Type) types.Type {
 	typ.SetName(name)
 	m.TypeDefs = append(m.TypeDefs, typ)