Summary:
D2917 added a change that will make paths on Windows response files
use DOS 8.3 shortnames to get around the fact that `libiberty` assumes
a one byte per character encoding.

This is actually not the problem, the actual problem is that GCC on
Windows doesn't seem to support Unicode at all.

This comes down to how unicode characters are handled between POSIX and
Windows. On Windows, Unicode is only supported using a multibyte character
encoding such as `wchar_t` with calls to the appropriate wide version of
APIs (name post-fixed with the `W` character). On Posix I believe the standard
`char` is used and based on the value it is decoded to the correct string.

GCC doesn't seem to make calls to the Wide version of the Windows APIs,
and even if it did, it's character representation would be wrong. So I
believe GCC just does not support utf-8 paths on Windows.

So the hack in D2917 is the only way to get Unicode support. The problem is
however that `GCC` is not the only tool with this issue and we don't use response
files for every invocation of the tools. Most of the tools probably don't support it.

Furthermore, DOS 8.1 shortnames only exist when the path or file physically exists on
disk. We pass lots of paths to GCC that don't exist yet, like the output file.
D2917 works around this by splitting the path from the file and try shortening that.

But this may not always work.

In short, even if we do Unicode correctly (which we don't atm, the GCC driver we build
uses `char` instead of `wchar_t`) we won't be able to compile using unicode paths that
need to be passed to `GCC`. So not sure about the point of D2917.

What we can do is support the most common non-ascii characters by writing the response
files out using the `latin1` code page.

Test Plan: compile + make test TEST=T12971

Reviewers: austin, bgamari, erikd

Reviewed By: bgamari

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2942

GHC Trac Issues: #12971