[clang] [Clang]Throw frontend error for target feature mismatch when using `flatten` attribute (PR #150044)

[Aaron Ballman via cfe-commits]

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@@ -287,6 +287,10 @@ def err_function_needs_feature : Error<
   "always_inline function %1 requires target feature '%2', but would "
   "be inlined into function %0 that is compiled without support for '%2'">;
 
+def err_flatten_function_needs_feature
+    : Error<"flatten function %0 calls %1 which requires target feature '%2', "
+            "but the caller is compiled without support for '%2'">;
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AaronBallman wrote:

@efriedma-quic at what point should we consider making `DiagnosticCodeGenKinds.td`? The diagnostics for codegen seem to be sprinkled all over the place.

https://github.com/llvm/llvm-project/pull/150044