Fix Windows LLVM to version 15 until bazel 6.4

[corco]

Bazel fails with LLVM 16, which is now installed on Windows CI.

Fix version 15 for now.

The issue is resolved in bazel, but not released yet.

[codecov-commenter]

Codecov Report

Patch coverage: 92.12% and project coverage change: -0.01% ⚠️

Comparison is base (0b8cb4e) 92.85% compared to head (7df0ef1) 92.85%.
Report is 11 commits behind head on master.

❗ Current head 7df0ef1 differs from pull request most recent head b496c2e. Consider uploading reports for the commit b496c2e to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2014      +/-   ##
==========================================
- Coverage   92.85%   92.85%   -0.01%     
==========================================
  Files         355      357       +2     
  Lines       26272    26399     +127     
==========================================
+ Hits        24395    24512     +117     
- Misses       1877     1887      +10     

Files Changed	Coverage Δ
common/util/sha256.cc	92.00% <92.00%> (ø)
common/util/sha256.h	100.00% <100.00%> (ø)

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[hzeller]

Fixes #2013 - thanks @corco

I'll wait for the Windows build to pass, then merge.