[clang] [Format] Fix detection of languages when reading from stdin (PR #79051)

[Björn Schäpers via cfe-commits]

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@@ -0,0 +1,5 @@
+// RUN: clang-format -dump-config - < %s | FileCheck %s
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HazardyKnusperkeks wrote:

I don't think you need this, right? The unit tests does the same, or not?

https://github.com/llvm/llvm-project/pull/79051