[build, darwin] Update CircleCI to Xcode 10.2.1

[friedbunny]

Update CircleCI builds to Xcode 10.2.1 — container specs.

/cc @julianrex

[friedbunny]

Going to skip updating the node-macos-release job, per #14651.

[friedbunny]

The check-public-symbols script was failing with ENOBUFS because the new CircleCI container updated to NodeJS 12.1.0 (from 11.13.0), which enforces a default buffer limit for execFileSync().

Since we were piping a large JSON file from SourceKitten, we overran the small default buffer — cd24f4c sets the buffer back to Infinity. This change is tested to be backwards compatible with Node 8, 10, and 11.

$ #!/bin/bash -eo pipefail
make darwin-check-public-symbols
^D��^D��node platform/darwin/scripts/check-public-symbols.js macOS iOS
Installing SourceKitten via Homebrew…
child_process.js:652
    throw err;
    ^

Error: spawnSync sourcekitten ENOBUFS
    at Object.spawnSync (internal/child_process.js:1020:20)
    at spawnSync (child_process.js:616:24)
    at execFileSync (child_process.js:644:15)
    at hasMissingSymbols (/Users/distiller/project/platform/darwin/scripts/check-public-symbols.js:21:16)
    at Object.<anonymous> (/Users/distiller/project/platform/darwin/scripts/check-public-symbols.js:72:16)
    at Module._compile (internal/modules/cjs/loader.js:759:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:770:10)
    at Module.load (internal/modules/cjs/loader.js:628:32)
    at Function.Module._load (internal/modules/cjs/loader.js:555:12)
    at Function.Module.runMain (internal/modules/cjs/loader.js:824:10)
make: *** [darwin-check-public-symbols] Error 1
Exited with code 2