When building with `-DLLVM_WINDOWS_PREFER_FORWARD_SLASH=ON`, tools like
lld output paths with forward slashes on Windows. However, lit's default
substitutions (`%t`, `%p`) typically use backslashes on Windows, causing
FileCheck failures in tests that strictly match path separators.
This patch propagates the `LLVM_WINDOWS_PREFER_FORWARD_SLASH` build flag
to llvm-lit via `builtin_parameters`. It also updates lit's TestRunner
to respect the 'use_normalized_slashes' parameter. When enabled, lit
normalizes paths in substitutions to use forward slashes, ensuring that
test expectations align with the tool output.
With this fix, the number of failed tests with
`-DLLVM_WINDOWS_PREFER_FORWARD_SLASH=ON` changes as follow:
- The total number of failed tests: 303 -> 168
- Break down:
- `Builtins-i386-windows` tests: 99 -> 0
- `Clang` tests: 28 -> 5
- `Clang Tools` tests: 7 -> 1
- `LLVM` tests: 6 -> 4
- `lld` tests: 2 -> 0
- Other failed tests:
- `glang`: 1
- `Clang-Unit`: 5
- `Clangd`: 3
- `Clangd Unit Tests`: 150
- `LLVM-Unit`: 1
4ea39c43e1