[Backtracing] Use reserveCapacity to optimize escaping slightly.

We know we need at least as much space as the number of UTF-8
characters.

rdar://121430255

Author: al45tair

stdlib/public/libexec/swift-backtrace/Utils.swift

@@ -264,6 +264,8 @@ func escapeJSON(_ s: String) -> String {
   var pos = chunk
   let end = utf8View.endIndex
 
+  result.reserveCapacity(utf8View.count)
+
   while pos != end {
     let scalar = utf8View[pos]
     switch scalar {