[clang-tools-extra] [clang-tidy] Rename 'cert-dcl50-cpp' to 'modernize-avoid-variadic-functions' (PR #157737)

[Baranov Victor via cfe-commits]

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+.. title:: clang-tidy - modernize-avoid-variadic-functions
+
+modernize-avoid-variadic-functions
+==================================
+
+Find all function definitions (but not declarations) of C-style variadic
+functions.
+
+Instead of C-style variadic functions, C++ function parameter pack or currying
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vbvictor wrote:

~Currying means making a curry.. nothing more!~
It's not widely used in C++ but its idea to transform `f(a, b, c ..)` to `f(a)(b)(c)..()`. https://en.wikipedia.org/wiki/Currying. It's more used in functional languages.

https://github.com/llvm/llvm-project/pull/157737