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

[Ben Hamilton via cfe-commits]

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

In this case, we do want two tests (at least, we do need the integration test in `clang/test/Format/dump-config-objc-stdin.m` to reproduce the issue).

The reason is, the bug was introduced into `ClangFormat.cpp`, which is only linked into the `clang-format` binary executed by the integration test. The code which regressed is not actually part of the `Format` library which is executed via the unit tests.

The unit tests do not execute the `clang-format` binary; they directly execute the formatting / config dumping logic via the `Format` library. But they're much faster to build and run, and it's possible this might regress in the `Format` library in the future (I was just surprised there were no tests there).

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