cmd/cgo: add #line directives to avoid printing bogus references to Go source files

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>

src/cmd/cgo/ast.go

@@ -87,6 +87,7 @@ func (f *File) ReadGo(name string) {
 			if cg != nil {
 				f.Preamble += fmt.Sprintf("#line %d %q\n", sourceLine(cg), name)
 				f.Preamble += commentText(cg) + "\n"
+				f.Preamble += "#line 1 \"cgo-generated-wrapper\"\n"
 			}
 		}
 	}

src/cmd/cgo/gcc.go

@@ -429,6 +429,7 @@ func (p *Package) loadDWARF(f *File, names []*Name) {
 	var b bytes.Buffer
 	b.WriteString(f.Preamble)
 	b.WriteString(builtinProlog)
+	b.WriteString("#line 1 \"cgo-dwarf-inference\"\n")
 	for i, n := range names {
 		fmt.Fprintf(&b, "__typeof__(%s) *__cgo__%d;\n", n.C, i)
 		if n.Kind == "const" {

src/cmd/cgo/out.go

@@ -126,6 +126,13 @@ func (p *Package) writeDefs() {
 		fmt.Fprint(fgo2, goProlog)
 	}
 
+	if fc != nil {
+		fmt.Fprintf(fc, "#line 1 \"cgo-generated-wrappers\"\n")
+	}
+	if fm != nil {
+		fmt.Fprintf(fm, "#line 1 \"cgo-generated-wrappers\"\n")
+	}
+
 	gccgoSymbolPrefix := p.gccgoSymbolPrefix()
 
 	cVars := make(map[string]bool)
@@ -1301,6 +1308,7 @@ func (p *Package) cgoType(e ast.Expr) *Type {
 }
 
 const gccProlog = `
+#line 1 "cgo-gcc-prolog"
 /*
   If x and y are not equal, the type will be invalid
   (have a negative array count) and an inscrutable error will come
@@ -1334,6 +1342,7 @@ const noTsanProlog = `
 
 // This must match the TSAN code in runtime/cgo/libcgo.h.
 const yesTsanProlog = `
+#line 1 "cgo-tsan-prolog"
 #define CGO_NO_SANITIZE_THREAD __attribute__ ((no_sanitize_thread))
 
 long long _cgo_sync __attribute__ ((common));
@@ -1356,6 +1365,7 @@ static void _cgo_tsan_release() {
 var tsanProlog = noTsanProlog
 
 const builtinProlog = `
+#line 1 "cgo-builtin-prolog"
 #include <stddef.h> /* for ptrdiff_t and size_t below */
 
 /* Define intgo when compiling with GCC.  */
@@ -1508,6 +1518,7 @@ func (p *Package) cPrologGccgo() string {
 }
 
 const cPrologGccgo = `
+#line 1 "cgo-c-prolog-gccgo"
 #include <stdint.h>
 #include <stdlib.h>
 #include <string.h>
@@ -1605,6 +1616,7 @@ func (p *Package) gccExportHeaderProlog() string {
 
 const gccExportHeaderProlog = `
 /* Start of boilerplate cgo prologue.  */
+#line 1 "cgo-gcc-export-header-prolog"
 
 #ifndef GO_CGO_PROLOGUE_H
 #define GO_CGO_PROLOGUE_H
@@ -1658,6 +1670,7 @@ const gccExportHeaderEpilog = `
 // We use weak declarations, and test the addresses, so that this code
 // works with older versions of gccgo.
 const gccgoExportFileProlog = `
+#line 1 "cgo-gccgo-export-file-prolog"
 extern _Bool runtime_iscgo __attribute__ ((weak));
 
 static void GoInit(void) __attribute__ ((constructor));