Add benchmarks for Substring.dropFirst(_:) and dropLast(_:) (#32473)

* Add benchmarks for Substring.dropFirst(_:) and dropLast(_:)

These are self-slicing operations, so they should be fast even for
this non-random-access collection.

* Fiddle with the iteration count and length

Author: natecook1000

benchmark/single-source/Substring.swift

@@ -17,6 +17,8 @@ public let SubstringTest = [
   BenchmarkInfo(name: "EqualSubstringString", runFunction: run_EqualSubstringString, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "EqualSubstringSubstring", runFunction: run_EqualSubstringSubstring, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "EqualSubstringSubstringGenericEquatable", runFunction: run_EqualSubstringSubstringGenericEquatable, tags: [.validation, .api, .String]),
+  BenchmarkInfo(name: "SubstringDropFirst1", runFunction: run_SubstringDropFirst1, tags: [.validation, .api, .String]),
+  BenchmarkInfo(name: "SubstringDropLast1", runFunction: run_SubstringDropLast1, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "LessSubstringSubstring", runFunction: run_LessSubstringSubstring, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "LessSubstringSubstringGenericComparable", runFunction: run_LessSubstringSubstringGenericComparable, tags: [.validation, .api, .String]),
   BenchmarkInfo(name: "StringFromLongWholeSubstring", runFunction: run_StringFromLongWholeSubstring, tags: [.validation, .api, .String]),
@@ -34,6 +36,8 @@ let longWide = "fὢasὢodὢijὢadὢolὢsjὢalὢsdὢjlὢasὢdfὢijὢ
 let (s1, ss1) = equivalentWithDistinctBuffers()
 let (s2, ss2) = equivalentWithDistinctBuffers()
 
+let quiteLong = String(repeating: "0", count: 15_000)[...]
+
 @inline(never)
 public func run_SubstringFromLongString(_ N: Int) {
   var s = longWide
@@ -126,6 +130,22 @@ public func run_EqualSubstringSubstringGenericEquatable(_ N: Int) {
   }
 }
 
+@inline(never)
+public func run_SubstringDropFirst1(_ N: Int) {
+  let s = quiteLong
+  for _ in 1...N*1000 {
+    blackHole(!s.dropFirst(1).isEmpty)
+  }
+}
+
+@inline(never)
+public func run_SubstringDropLast1(_ N: Int) {
+  let s = quiteLong
+  for _ in 1...N*1000 {
+    blackHole(!s.dropLast(1).isEmpty)
+  }
+}
+
 /*
 func checkEqual<T, U>(_ x: T, _ y: U)
 where T : StringProtocol, U : StringProtocol {