[clang] [Clang] Don't crash if input file is not a module. (PR #98439)

[Chuanqi Xu via cfe-commits]

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@@ -226,6 +226,7 @@ def err_module_map_not_found : Error<"module map file '%0' not found">,
 def err_missing_module_name : Error<
   "no module name provided; specify one with -fmodule-name=">,
   DefaultFatal;
+def err_file_is_not_module : Error<"file '%0' is not a module">, DefaultFatal;
----------------
ChuanqiXu9 wrote:

```suggestion
def err_file_is_not_module : Error<"file '%0' is not a module file">, DefaultFatal;
```

https://github.com/llvm/llvm-project/pull/98439