fatal error: caching backend error: cache poisoned for ...

### Description We encountered a CAS poisoned error when building llbuild. ``` FAILED: [code=1] unittests/BuildSystem/CMakeFiles/BuildSystemTests.dir/TempDir.cpp.obj S:\b\toolchains\DEVELOPMENT-SNAPSHOT-2026-02-06-a\LocalApp\Programs\Swift\Toolchains\0.0.0+Asserts\usr\bin\clang-cache S:\b\toolchains\DEVELOPMENT-SNAPSHOT-2026-02-06-a\LocalApp\Programs\Swift\Toolchains\0.0.0+Asserts\usr\bin\clang++.exe --target=x86_64-unknown-windows-msvc -DGTEST_HAS_PTHREAD=0 -DLLVM_ON_WIN32 -IS:/SourceCache/llbuild/include -IS:/SourceCache/llbuild/utils/unittest/googletest/include -IS:/SourceCache/llbuild/utils/unittest/googlemock/include -isystem S:/SourceCache/swift-toolchain-sqlite/Sources/CSQLite/include -fno-stack-protector -ffunction-sections -fdata-sections -fomit-frame-pointer -finline-functions -O3 -DNDEBUG -std=c++14 -D_DLL -D_MT -Xclang --dependent-lib=msvcrt -fno-rtti -fno-exceptions -Wbool-conversion -Wconstant-conversion -Wdeprecated-declarations -Wdocumentation -Wempty-body -Wenum-conversion -Wimplicit-fallthrough -Wint-conversion -Winvalid-offsetof -Wmost -Wnewline-eof -Wnon-virtual-dtor -Woverloaded-virtual -Wparentheses -Wsign-compare -Wswitch -Wuninitialized -Wunused-function -Wunused-value -Wunused-variable -Wunreachable-code -MD -MT unittests/BuildSystem/CMakeFiles/BuildSystemTests.dir/TempDir.cpp.obj -MF unittests\BuildSystem\CMakeFiles\BuildSystemTests.dir\TempDir.cpp.obj.d -o unittests/BuildSystem/CMakeFiles/BuildSystemTests.dir/TempDir.cpp.obj -c S:/SourceCache/llbuild/unittests/BuildSystem/TempDir.cpp fatal error: caching backend error: cache poisoned for 'eece0e5a96cd2a9fab9e47241a2e624ef2d04cb85cb1e3d5b913b2a6656ba4b7' (new='llvmcas://4bf27fbadc47b43ecc9a5e9e86e6462a3e10c943a7260044a35842ae973f205d' vs. existing 'llvmcas://3e86b0cab1b34cb57188e22fd42db2c2358801dd1dbf0b985a71169bf2633cde') ``` ### Reproduction With a recent build of the toolchain locally built or installed (from such as [this](https://download.swift.org/development/windows10/swift-DEVELOPMENT-SNAPSHOT-2026-02-06-a/swift-DEVELOPMENT-SNAPSHOT-2026-02-06-a-windows10.exe)), run the following ``` git clone http://github.com/swiftlang/swift-llbuild cd swift-llbuild mkdir build cd build set LLVM_CACHE_CAS_PATH=C:\cas-llbuild cmake -S .. -G Ninja -D BUILD_SHARED_LIBS=YES -D CMAKE_BUILD_TYPE=Release -D CMAKE_CXX_COMPILER=clang++.exe -D CMAKE_CXX_COMPILER_LAUNCHER=clang-cache ninja ``` ### Expected behavior No error ### Environment The main branch on Windows ### Additional information _No response_