[cxx-interop] Increase the size of std::strings in benchmarks

Since #75608, the performance of `std::string` <=> `Swift.String` conversions improved significantly. To make sure the workload is significant enough for the benchmark results to be noise-free, this bumps the size of Swift strings that are being tested.

Author: egorzhdan

benchmark/cxx-source/CxxStringConversion.swift

@@ -15,7 +15,7 @@ import CxxStdlibPerformance
 import CxxStdlib
 
 let cxxStringSize = 1_000_000
-let swiftStringSize = 25_000
+let swiftStringSize = 1_000_000
 
 var cxxString: std.string? = nil
 var swiftString: String? = nil