cmd/cgo: read source files once

Now cgo reads source files twice: for c prefix generation and parsing
go code to an ast node. It can be narrowed down to single loop.

Change-Id: Ie05452a3a12106aaab863244727390037e69e8e6
Reviewed-on: https://go-review.googlesource.com/40939


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>