cmd/cgo: don't let #cgo directives mess up line numbering

Fixes #5272.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/13498046

misc/cgo/errors/err1.go

+package main
+
+/*
+#cgo LDFLAGS: -c
+
+void test() {
+	xxx;		// This is line 7.
+}
+*/
+import "C"
+
+func main() {
+	C.test()
+}

misc/cgo/errors/test.bash

+# Copyright 2013 The Go Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+if go tool cgo err1.go >errs 2>&1; then
+  echo 1>&2 misc/cgo/errors/test.bash: BUG: expected cgo to fail but it succeeded
+  exit 1
+fi
+if ! test -s errs; then
+  echo 1>&2 misc/cgo/errors/test.bash: BUG: expected error output but saw none
+  exit 1
+fi
+if ! fgrep err1.go:7 errs >/dev/null 2>&1; then
+  echo 1>&2 misc/cgo/errors/test.bash: BUG: expected error on line 7 but saw:
+  cat 1>&2 errs
+  exit 1
+fi
+rm -rf errs _obj
+exit 0

src/cmd/cgo/gcc.go

@@ -76,6 +76,8 @@ func (f *File) DiscardCgoDirectives() {
 		l := strings.TrimSpace(line)
 		if len(l) < 5 || l[:4] != "#cgo" || !unicode.IsSpace(rune(l[4])) {
 			linesOut = append(linesOut, line)
+		} else {
+			linesOut = append(linesOut, "")
 		}
 	}
 	f.Preamble = strings.Join(linesOut, "\n")

src/run.bash

@@ -145,6 +145,12 @@ esac
 go run main.go || exit 1
 ) || exit $?
 
+[ "$CGO_ENABLED" != 1 ] ||
+[ "$GOHOSTOS" == windows ] ||
+(xcd ../misc/cgo/errors
+./test.bash || exit 1
+) || exit $?
+
 (xcd ../doc/progs
 time ./run || exit 1
 ) || exit $?