cgo: add support for C typedefs

testdata/cgo/main.go

@@ -4,10 +4,13 @@ package main
 #include <stdint.h>
 int32_t fortytwo(void);
 int32_t mul(int32_t a, int32_t b);
+typedef int32_t myint;
 */
 import "C"
 
 func main() {
 	println("fortytwo:", C.fortytwo())
-	println("mul:", C.mul(int32(3), 5))
+	println("mul:", C.mul(C.int32_t(3), 5))
+	var x C.myint = 3
+	println("myint:", x, C.myint(5))
 }

testdata/cgo/out.txt

@@ -1,2 +1,3 @@
 fortytwo: 42
 mul: 15
+myint: 3 5