[cs] re-order initialization

[Zack Weinberg]

Per Bothner <per@bothner.com> writes:

> Worth noting:  The assembler output file is initialized
> and opened *before* we call cpp_read_main_file.  This may
> be controversial: the code no longer looks for an initial
> #line directive in the input file to set the main_input_filename.
> However, doing so was a gross hack that seems pointless now that
> we have an integrated cpplib.  For the few cases where you
> really need to compile already-pre-processed source file, and
> you for some reason care about the main_input_filename used,
> then you can specify --dumpbase.

The primary purpose of this was to (attempt to) get the behavior to
change as little as possible with -save-temps.  It's probably also a
concern with ccache and distcc.

I would be delighted to see that gross hack go away on the mainline,
if you can make -save-temps DTRT and coordinate with the ccache/distcc
people so they still get the behavior they want.

zw