Add simple benchmark for append(contentsOf:)

[airspeedswift]

Adding this simple benchmark in preparation for a handful of ABI-related refactoring changes.

There is room for improvement in these tests, as well as the original single-element appends on which they're based, in terms of:

• picking the actual number of times/elements to append on a more principled basis (current values are arbitrary)
• testing multiple possible types in the array:
  • 1 swift class.
  • struct with 2 swift class.
  • enum with single trivial payload
  • enum with single non-trivial payload
  • enum with multiple trivial payloads
  • enum with multiple non-trivial payloads
  • objective-c class
  • Existential Box.
  • Unspecialized Generic Type

all of which may have different performance characteristics.

But for now this should serve to indicate if most planned changes have significant perf impact.

[airspeedswift]

@swift-ci please benchmark

[swift-ci]

Build comment file:

Optimized (O)

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[airspeedswift]

Appears benchmarking comparison script can't handle new benchmarks. But aside from that looks to be working correctly (rdar://problem/28942824).

@swift-ci please smoke test and merge

[shahmishal]

@swift-ci please smoke test and merge