[clang-tools-extra] [clang-tidy] Emit warnings from user headers by default (PR #164165)

[Carlos Galvez via cfe-commits]

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@@ -133,13 +133,15 @@ file, if any.
 static cl::opt<std::string> HeaderFilter("header-filter", desc(R"(
 Regular expression matching the names of the
 headers to output diagnostics from. Diagnostics
-from the main file of each translation unit are
-always displayed.
+from the main file and all non-system headers 
+of each translation unit are always displayed.
+Set this option to an empty string to disable 
+diagnostics from non-system headers.
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carlosgalvezp wrote:

When reading it, I still think there can be confusion. One could interpret the docs as "there's this flag to filter headers, but non-system headers are always included, regardless".

Maybe something like this could eliminate the possible confusion: "The default value is `.*`, i.e. diagnostics from all non-system headers are displayed by default".

https://github.com/llvm/llvm-project/pull/164165