github: Stop disabling LLVM_WINDOWS_PREFER_FORWARD_SLASH in test-llvm

Since LLVM 23, the testcases should pass with
LLVM_WINDOWS_PREFER_FORWARD_SLASH enabled. For mingw build
configurations, this is enabled by default.

.github/workflows/test-llvm.yml

@@ -84,7 +84,6 @@ jobs:
             -DCMAKE_SYSTEM_NAME=Windows `
             -DLLVM_TARGETS_TO_BUILD="AArch64;X86" `
             -DLLVM_ENABLE_PROJECTS="clang;lld;clang-tools-extra" `
-            -DLLVM_WINDOWS_PREFER_FORWARD_SLASH=OFF `
             -DCMAKE_C_COMPILER=clang `
             -DCMAKE_CXX_COMPILER=clang++ `
             -DCMAKE_CXX_FLAGS="-D__USE_MINGW_ANSI_STDIO=${{startsWith(matrix.arch, 'a') && '0' || '1'}}" `