[clang] [clang] Support header shadowing diagnostics in Clang header search (PR #162491)

[Qiu Chaofan via cfe-commits]

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@@ -789,6 +789,8 @@ def ShadowFieldInConstructor : DiagGroup<"shadow-field-in-constructor",
 def ShadowIvar : DiagGroup<"shadow-ivar">;
 def ShadowUncapturedLocal : DiagGroup<"shadow-uncaptured-local">;
 
+def HeaderShadowing : DiagGroup<"header-shadowing">;
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ecnelises wrote:

Maybe better as ShadowHeader to fit into convention here?

https://github.com/llvm/llvm-project/pull/162491