BridgeJS: Perf-tune SwiftToSkeleton by avoiding unnecessary Syntax node allocation

[kateinoigakukun]

trimmedDescription allocates two new Syntax nodes just to get the trimmed description of a node.

extension SyntaxProtocol {
  public var trimmedDescription: String {
    // TODO: We shouldn't need to create to copies just to get the trimmed
    // description.
    return self.trimmed.description
  }
  public var trimmed: Self {
    // TODO: Should only need one new node here
    return self.with(\.leadingTrivia, []).with(\.trailingTrivia, [])
  }
}

It speeds up 4x faster

$ cat check.d.ts
export function getCanvas(x: string): HTMLCanvasElement;

$ node Plugins/BridgeJS/Sources/TS2Swift/JavaScript/bin/ts2swift.js check.d.ts -p tsconfig.json -o out.swift
$ hyperfine "./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.before emit-skeleton out.swift" "./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.after emit-skeleton out.swift"
Benchmark 1: ./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.before emit-skeleton out.swift
  Time (mean ± σ):     16.531 s ±  0.317 s    [User: 16.355 s, System: 0.107 s]
  Range (min … max):   16.026 s … 17.252 s    10 runs

Benchmark 2: ./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.after emit-skeleton out.swift
  Time (mean ± σ):      4.068 s ±  0.134 s    [User: 3.976 s, System: 0.039 s]
  Range (min … max):    4.000 s …  4.445 s    10 runs

Summary
  ./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.after emit-skeleton out.swift ran
    4.06 ± 0.15 times faster than ./Plugins/BridgeJS/.build/debug/BridgeJSToolInternal.before emit-skeleton out.swift