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

[Qiu Chaofan via cfe-commits]

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@@ -557,6 +557,10 @@ def warn_pp_hdrstop_filename_ignored : Warning<
 def remark_pp_search_path_usage : Remark<
   "search path used: '%0'">,
   InGroup<UsedSearchPath>;
+def warn_header_shadowed
+  : Warning<"multiple candidates for header '%0' found; "
+            "using the one from '%1', shadowed by '%2'">,
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ecnelises wrote:

When we have `dir1/a.h` and `dir2/a.h` and `dir1` has higher priority, is `shadowed by 'dir2'` a bit confusing?

Maybe as ...

> multiple candidates for header 'a.h' found; directory 'dir1' chosen, ignoring 'dir2'

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