cmd/cgo: revert CL 49490 "fix for function taking pointer typedef"

CL 49490 fixed a warning when compiling the C code generated by cgo,
but it introduced typedef conflicts in Go code that cgo is supposed to
avoid.

Original CL description:

    cmd/cgo: fix for function taking pointer typedef

    Fixes #19832

Updates #19832
Fixes #23720

Change-Id: I22a732db31be0b4f7248c105277ab8ee44ef6cfb
Reviewed-on: https://go-review.googlesource.com/92455
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

misc/cgo/test/issue19832.go

-// Copyright 2015 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.
-
-// Issue 19832. Functions taking a pointer typedef were being expanded and triggering a compiler error.
-
-package cgotest
-
-// typedef struct { int i; } *PS;
-// void T19832(PS p) {}
-import "C"
-import "testing"
-
-func test19832(t *testing.T) {
-	C.T19832(nil)
-}

src/cmd/cgo/gcc.go

@@ -2250,12 +2250,6 @@ func (c *typeConv) FuncArg(dtype dwarf.Type, pos token.Pos) *Type {
 				break
 			}
 
-			// If we already know the typedef for t just use that.
-			// See issue 19832.
-			if def := typedef[t.Go.(*ast.Ident).Name]; def != nil {
-				break
-			}
-
 			t = c.Type(ptr, pos)
 			if t == nil {
 				return nil