[clang] [Preprocessor] Do not expand macros if the input is already preprocessed (PR #137665)

[Juan Manuel Martinez Caamaño via cfe-commits]

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@@ -48,6 +48,7 @@
 // instantiation for v<int> in one of the two headers, because we will only
 // parse one of the two get() functions.
 
+#ifdef NS
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jmmartinez wrote:

I'll double check this next week and bring a good explanation.

I remember that the test didn't work since the file-type of the input file was marked as already preprocessed (so the there were issues with the `#if NS` directive).

https://github.com/llvm/llvm-project/pull/137665