[clang-tools-extra] [clang-tidy] Add readability-use-span-first-last check (PR #118074)

[Julian Schmidt via cfe-commits]

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+.. title:: clang-tidy - readability-use-span-first-last
+
+readability-use-span-first-last
+===============================
+
+Checks for uses of ``std::span::subspan()`` that can be replaced with clearer
+``first()`` or ``last()`` member functions. These dedicated methods were added 
+to C++20 to provide more expressive alternatives to common subspan operations.
+
+Covered scenarios:
+
+========================== ====================
+Expression                 Replacement
+------------------------- --------------------
+``s.subspan(0, n)``       ``s.first(n)``
+``s.subspan(s.size() - n)`` ``s.last(n)``
+========================== ====================
+
+
+Non-zero offset with count (like ``subspan(1, n)``) or offset-only calls 
+(like ``subspan(n)``) have no clearer equivalent using ``first()`` or 
+``last()``, so these cases are not transformed.
+
+This check is only active when C++20 or later is used.
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5chmidti wrote:

Missing newline

https://github.com/llvm/llvm-project/pull/118074