cmd/cgo: don't define intgo in export prologue

The export prologue goes into the _cgo_export.h file, where it may be
be #include'd by a .swig file. As SWIG defines its own type "intgo",
the definition of "intgo" in the export prologue could conflict.
Since we don't need to define "intgo" in the _cgo_export.h file, don't.

Defining "intgo" in _cgo_export.h was new for this release, so this
should not break any existing code.

No test case as I can't quite bring myself to write a test that
combines SWIG and cgo.

Change-Id: I8073e8300a1860cecd5994b9ad07dd35a4298c89
Reviewed-on: https://go-review.googlesource.com/83936
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/cgo/out.go

@@ -1652,9 +1652,7 @@ const builtinExportProlog = `
 #ifndef GO_CGO_EXPORT_PROLOGUE_H
 #define GO_CGO_EXPORT_PROLOGUE_H
 
-typedef ptrdiff_t intgo;
-
-typedef struct { const char *p; intgo n; } _GoString_;
+typedef struct { const char *p; ptrdiff_t n; } _GoString_;
 
 #endif
 `