fix: strip SCCACHE_BASEDIRS from escaped-backslash paths on Windows

[rconde01]

fix: strip SCCACHE_BASEDIRS from escaped-backslash paths on Windows

Fixes #2737.

Problem

SCCACHE_BASEDIRS has no effect on Windows. The same source compiled
from two different checkout directories produces different cache keys, so
nothing is shared across roots — the feature silently does nothing.

Root cause

On Windows, paths inside preprocessor output are emitted as C string
literals. MSVC #line directives and clang GNU linemarkers quote them
with escaped backslash separators:

#line 1 "C:\\Users\\me\\project\\src\\main.c"

normalize_win_path converts every backslash to a forward slash, so each
escaped separator \\ becomes a double forward slash:

c://users//me//project//src//main.c

But the normalized basedir uses single slashes (c:/users/me/project/),
so strip_basedirs never finds a boundary match, and the absolute path is
left in the preprocessed output that feeds the hash.

Fix

In strip_basedirs, also search for a doubled-separator variant of each
basedir, produced by a new double_path_separators helper. Escaped paths
are now stripped in addition to the existing plain form; mixed
escaped/plain occurrences and UNC basedirs are handled too.

The extra needle and the helper are gated behind
cfg(target_os = "windows"), so non-Windows builds and behavior are
unchanged.

Tests

New unit tests (Windows-only):

• util::tests::test_strip_basedir_windows_escaped_backslashes — MSVC
  #line, clang linemarker, and mixed escaped/plain occurrences.
• util::tests::test_strip_basedir_windows_escaped_unc — escaped UNC
  basedir.
• compiler::c::test::test_hash_key_basedirs_windows_escaped_backslashes
  — the same source under two different basedir roots hashes equally, and
  differs without basedirs.

All three fail before the change and pass after. cargo test --lib,
cargo fmt --check, and cargo clippy --lib are clean on the changed
code (the single pre-existing clippy warning in msvc.rs is untouched).

Manual end-to-end verification

Reproduced with real toolchains on Windows: building the same source from
two distinct absolute directories with SCCACHE_BASEDIRS set goes from
0/2 cache hits (unpatched) to 1/2 (patched) for both clang and
MSVC.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.57%. Comparing base (5d52f91) to head (7fb7508).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2736      +/-   ##
==========================================
+ Coverage   74.51%   74.57%   +0.05%     
==========================================
  Files          70       70              
  Lines       39652    39737      +85     
==========================================
+ Hits        29546    29632      +86     
+ Misses      10106    10105       -1     

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[sylvestre]

in the future, please keep only the most important information in comment #0