go/types: support cgo semantics

[hirochachacha]

I was thinking about #975.
My idea is guessing argument types and making specialized wrappers.
For example,
We can convert following code

package main

// #include <stdio.h>
import "C"

func main() {
	C.printf(C.CString("hello: %d"), 10)
	C.fflush(C.stdout)
}

into

package main

// #include <stdio.h>
// int call_printf_with_int(const char* s, int i) {
//   return printf(s, i);
// }
import "C"

func main() {
	C.call_printf_with_int(C.CString("hello: %d"), 10)
	C.fflush(C.stdout)
}

I'd like to leave these translations to cgo.

To promote this idea, we have to make go/types understand some special cgo semantics.

• all cgo objects are exportable even if the first letter is lower case (e.g. C.char, C.int, ...)
• cgo functions behave like unsafe.Pointer in the expression context (e.g. var xxx unsafe.Pointer = C.puts)
• cgo functions can return the optional error argument in the assignment context (e.g _, err := C.puts(C.CString("x"))

Is this acceptable?

FWIW, https://go-review.googlesource.com/c/go/+/83215 is the attempt.

[ianlancetaylor]

CC @griesemer @mdempsky

[griesemer]

Too late for 1.11.