[Syntax] Add init(unsafeCasting: Syntax) to concrete node types

[rintaro]

init(unsafeCasting:) should only be used when we are absolutely sure the syntax kind matches the type, and only in perf critial places. Use it in SyntaxEnum, SyntaxVisitor, and SyntaxRewriter.

Also, rework SyntaxVisitor/SyntaxRewriter visitation, eliminating unnecessary ARC traffic.

Baseline:

'-[PerformanceTest.PerfTests testEmptyRewriter]' measured [Time, seconds] average: 1.626, relative standard deviation: 0.492%, values: [1.645482, 1.618930, 1.620801, 1.619107, 1.621778, 1.620863, 1.621619, 1.629248, 1.633502, 1.628365],
'-[PerformanceTest.PerfTests testEmptyVisitor]' measured [Time, seconds] average: 1.169, relative standard deviation: 0.825%, values: [1.195887, 1.166237, 1.164091, 1.167106, 1.167065, 1.175714, 1.164696, 1.160530, 1.163220, 1.167872]

With this patch:

'-[PerformanceTest.PerfTests testEmptyRewriter]' measured [Time, seconds] average: 1.281, relative standard deviation: 0.991%, values: [1.317752, 1.279236, 1.281419, 1.273362, 1.273581, 1.277479, 1.283182, 1.277359, 1.273263, 1.273686]
'-[PerformanceTest.PerfTests testEmptyVisitor]' measured [Time, seconds] average: 0.933, relative standard deviation: 0.710%, values: [0.952931, 0.931542, 0.933043, 0.932171, 0.931005, 0.929917, 0.931035, 0.930155, 0.931712, 0.929429]

This patch eliminates ARC during visitation funcs, but we still need to retain the parent Syntax.Info when constructing Syntax.

[rintaro]

@swift-ci Please test

[ahoppen]

LGTM. I would be curious to see the performance numbers before merging.

[rintaro]

As per offline discussion with Alex, renamed init(_unsafeCasting:) to init(unsafeCasting:)

[rintaro]

@swift-ci Please test