[clang] [clang-format] Make ignored files unformatted instead of empty. (PR #170416)

[via cfe-commits]

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@@ -701,9 +711,8 @@ int main(int argc, const char **argv) {
   }
 
   if (FileNames.empty()) {
-    if (isIgnored(AssumeFileName))
-      return 0;
-    return clang::format::format("-", FailOnIncompleteFormat);
+    return clang::format::format("-", FailOnIncompleteFormat,
+                                 isIgnored(AssumeFileName));
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owenca wrote:

> * The problem also persists with `clang-format ignored-file.cc`

Why should clang-format output the ignored files?

> * We also shouldn't always output the input file. When `--output-replacements-xml` is provided, we need to do different logic.

We can condition it on `!OutputXML` as shown in the updated suggested change above.

https://github.com/llvm/llvm-project/pull/170416