[benchmark] Add integer-to-string benchmark (#85209)

A companion to #85180.

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Author: xwu

benchmark/CMakeLists.txt

@@ -109,6 +109,7 @@ set(SWIFT_BENCH_MODULES
     single-source/IndexPathTest
     single-source/InsertCharacter
     single-source/IntegerParsing
+    single-source/IntegerToString
     single-source/Integrate
     single-source/IterateData
     single-source/Join