[benchmark] Add some benchmarks for String breadcrumbs (swiftlang#20769)

[benchmark] Add some benchmarks for String breadcrumbs

Author: lorentey

benchmark/CMakeLists.txt

@@ -42,6 +42,7 @@ set(SWIFT_BENCH_MODULES
     single-source/BinaryFloatingPointConversionFromBinaryInteger
     single-source/BinaryFloatingPointProperties
     single-source/BitCount
+    single-source/Breadcrumbs
     single-source/ByteSwap
     single-source/COWTree
     single-source/COWArrayGuaranteedParameterOverhead