Fix clang version parsing on Windows.

[polasek]

The official Windows release of Clang 22 changed the output format of -v.

$ clang -v
clang version 22.1.1 (https://github.com/llvm/llvm-project fef02d48c08db859ef83f84232ed78bd9d1c323a)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files\LLVM\bin

This makes version parsing fail, and in turn leads to failed compilation for basically any C++ code as the version is used for cxx_builtin_include_directories. Without the change in this PR, I get

cxx_builtin_include_directories = [
    ...
    "C:\\Program Files\\LLVM\\lib\\clang\\fef02d48c08db859ef83f84232ed78bd9d1c323a)\\include"
],

instead of "C:\\Program Files\\LLVM\\lib\\clang\\22\\include" in the autoconfigured toolchain.

Compiling

#include <iostream>
int main() { return 0; }

then fails with:

ERROR: C:/users/jan/code/rules_cc_clang_cl_llvm22_repro/BUILD:3:8: Compiling main.cc failed: absolute path inclusion(s) found in rule '//:test':
the source file 'main.cc' includes the following non-builtin files with absolute paths (if these are builtin files, make sure these paths are in your toolchain):
  'C:/Program Files/LLVM/lib/clang/22/include/yvals_core.h'
  'C:/Program Files/LLVM/lib/clang/22/include/vadefs.h'
  'C:/Program Files/LLVM/lib/clang/22/include/yvals_core.h'
...

I didn't find any tests for this or any easy way of adding them but please let me know if I missed something.

[armandomontanez]

Please use a shared variable rather than duplicating "clang version ".

[polasek]

Done.

[armandomontanez]

Thanks!

This will merge per the merge process pending additional approvals.