Track Windows 7 vs. delete-on-close fixes

[cuviper]

Rust #82605 reverted LLVM D81803, "Avoid setting the delete-on-close bit if a TempFile doesn't reside on a local drive," because that regressed Windows 7 in #81051.

That change was meant to fix LLVM PR42623, which also references Rust #54216. It would be nice if some Windows user could verify if that was fixed on a nightly where we still had the change, roughly 2021-01-13 through 2021-02-28. But since it left Windows 7 unable to create any archives, we decided that regression was worse than the fix.

LLVM 48378 also reported a Windows 7 regression that seems to be the same root cause. Perhaps there will be a new update that makes it all work for Windows 7, so we can have the best of both. Another possibility suggested in a compiler meeting was to use runtime platform detection, skipping D81803's code on Windows 7, but letting us still have the bug fix on newer Windows.

Alternatively, when Rust eventually decides to stop supporting Windows 7, we can just drop the revert / reapply D81803.

[cuviper]

Can some Windows 7 users try the fix mentioned on LLVM 48378? I suppose you could build LLVM's main branch directly, and try some llvm-ar commands as mentioned in #81051 (comment).

[moxian]

Can confirm that llvm-ar works as of llvm/llvm-project@8f62a80

> cmake --build .
<snip>
> ls > hello.txt
> .\Debug\bin\llvm-ar.exe r hello.a hello.txt
D:\work\trash\github\llvm\build\Debug\bin\llvm-ar.exe: warning: creating hello.a
> ls hello*


    Directory: D:\work\trash\github\llvm\build


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       2021-03-26     11:50          14468 hello.a
-a----       2021-03-26     11:49          14400 hello.txt