[clang-tools-extra] [clang-tidy][NFC] Don't store a redundant copy of `HeaderFileExtensions` in every check (PR #190302)

[Victor Chernyakin via cfe-commits]

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@@ -17,10 +18,10 @@ namespace {
 AST_POLYMORPHIC_MATCHER_P(isInHeaderFile,
                           AST_POLYMORPHIC_SUPPORTED_TYPES(FunctionDecl,
                                                           VarDecl),
-                          FileExtensionsSet, HeaderFileExtensions) {
+                          const FileExtensionsSet *, HeaderFileExtensions) {
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localspook wrote:

Since the point of this PR is reducing redundant copies, I figured I would get rid of the redundant copies in the matchers too. Unfortunately the matchers framework makes this a bit awkward...

https://github.com/llvm/llvm-project/pull/190302