[clang] [clang-format] Add .clang-format-ignore for ignoring files (PR #76327)

[Owen Pan via cfe-commits]

================
@@ -570,6 +571,70 @@ static int dumpConfig(bool IsSTDIN) {
   return 0;
 }
 
+// Check whether `FilePath` is ignored according to the nearest
+// .clang-format-ignore file based on the rules below:
+// - A blank line is skipped.
+// - Leading and trailing spaces of a line are trimmed.
+// - A line starting with a hash (`#`) is a comment.
+// - A non-comment line is a single pattern.
+// - The slash (`/`) is used as the directory separator.
+// - A pattern is relative to the directory of the .clang-format-ignore file (or
+//   the root directory if the pattern starts with a slash).
+// - A pattern is negated if it starts with a bang (`!`).
+static bool isIgnored(StringRef FilePath) {
+  using namespace llvm::sys::fs;
+  if (!is_regular_file(FilePath))
+    return false;
+
+  using namespace llvm::sys::path;
+  SmallString<128> Path, AbsPath{FilePath};
+
+  make_absolute(AbsPath);
+  remove_dots(AbsPath, /*remove_dot_dot=*/true);
+
+  StringRef IgnoreDir{AbsPath};
+  do {
+    IgnoreDir = parent_path(IgnoreDir);
+    if (IgnoreDir.empty())
+      return false;
+
+    Path = IgnoreDir;
+    append(Path, ".clang-format-ignore");
+  } while (!is_regular_file(Path));
+
+  std::ifstream IgnoreFile{Path.c_str()};
+  if (!IgnoreFile.good())
+    return false;
+
+  AbsPath = convert_to_slash(AbsPath);
+
----------------
owenca wrote:

```suggestion
  const auto Pathname = convert_to_slash(AbsPath);
```

https://github.com/llvm/llvm-project/pull/76327