msvc: fix detect_showincludes_prefix when VS_UNICODE_OUTPUT is set

[exoosh]

This reintroduces #1835 (FYI: @AudranDoublet). Copying the details over here:

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VS_UNICODE_OUTPUT is set by Visual Studio to a value that tells MS tools running from within the IDE where to send their output. Thus, cl.exe invocation used to retrieve the /showIncludes prefix returns an empty output when this variable is set to a valid value.

The proposed fix is to use /E option that tells cl.exe to output file content in stdout and retrieved headers in stderr. When set, cl.exe ignores the value of VS_UNICODE_OUTPUT.

Another fix could be to explicitly unset VS_UNICODE_OUTPUT in the command invocation.

#1830 #909 mentioning these issues as it may fix them.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.87%. Comparing base (764daee) to head (2f2e1f9).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/msvc.rs	0.00%	23 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (764daee) and HEAD (2f2e1f9). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (764daee)	HEAD (2f2e1f9)
	11	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2483       +/-   ##
===========================================
- Coverage   71.14%   27.87%   -43.27%     
===========================================
  Files          64       54       -10     
  Lines       35207    33494     -1713     
===========================================
- Hits        25047     9338    -15709     
- Misses      10160    24156    +13996     

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[exoosh]

Trying to tend to the failed checks. It also turns out that this fix does not address #1830 for me on VS2022 with cl.exe 19.43.34810, @AudranDoublet.

But your finding certainly is an excellent starting point for me to investigate further.

[exoosh]

Will rebase against main before this comes out of draft. Currently I rebased #2482 against main and added this change on top. The result seems to be a usable sccache.exe from the VS IDE.

[sylvestre]

most of the jobs are failing