A bit contrived to come up with an example, but it happened in #15836, somehow.

	$ cat /tmp/x.go
	package main

	/*
	#include <stddef.h>

	int foo(void);

	int foo(void) {
		return 2;
	}

	#define int asdf
	*/
	import "C"

	func main() {
		println(C.foo())
	}

	$ go run /tmp/x.go
	# command-line-arguments
	cgo-builtin-prolog:9:31: error: unknown type name 'asdf'   <<<<<
	_GoString_ GoStringN(char *p, int l);
	                              ^
	/tmp/x.go:12:13: note: expanded from macro 'int'
	#define int asdf
	            ^
	cgo-builtin-prolog:10:28: error: unknown type name 'asdf'  <<<<<
	_GoBytes_ GoBytes(void *p, int n);
	                           ^
	/tmp/x.go:12:13: note: expanded from macro 'int'
	#define int asdf
	            ^
	2 errors generated.

The two marked lines used to refer incorrectly to /tmp/x.go.

Fixes #15836.

Change-Id: I08ef60a53cfd148112fceb651eaf7b75d94a7a8d
Reviewed-on: https://go-review.googlesource.com/32613
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>