MSVC: support forward slash as an output dir marker

[AJIOB]

Fixes #2532

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.92308% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.08%. Comparing base (760a109) to head (4cdefd1).

Files with missing lines	Patch %	Lines
src/compiler/msvc.rs	76.92%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2535   +/-   ##
=======================================
  Coverage   71.08%   71.08%           
=======================================
  Files          64       64           
  Lines       35432    35457   +25     
=======================================
+ Hits        25186    25204   +18     
- Misses      10246    10253    +7     

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