Fix async arrow function parser bug #1989

Job	Run time
build-by-clang	5m 19s
build-test-api	6m 31s
build-on-macos	4m 53s
build-on-windows-x64-uwp-x86-shared	9m 50s
build-test-codecache	8m 24s
build-test-debugger	4m 9s
build-test-on-android (x86, 28)	26m 3s
build-test-on-android (x86_64, 28)	26m 37s
build-test-on-x64-release (new-es octane v8)	9m 51s
build-test-on-x64-release (new-es octane v8, -DESCARGOT_THREADING=ON -DESCARGOT_TCO=ON)	9m 40s
build-test-on-x64-release (chakracore spidermonkey)	7m 59s
build-test-on-x64-release (chakracore spidermonkey, -DESCARGOT_THREADING=ON -DESCARGOT_TCO=ON)	9m 18s
build-test-on-x64-release (chakracore spidermonkey, -DESCARGOT_SMALL_CONFIG=ON -DESCARGOT_USE_CUS...	10m 2s
build-test-on-x86-release (octane)	9m 14s
build-test-on-x86-release (v8 chakracore spidermonkey)	11m 57s
build-test-on-x86-release (jetstream-only-simple-parallel-1)	12m 39s
build-test-on-x86-release (jetstream-only-simple-parallel-2 jsc-stress)	7m 38s
build-test-on-x86-release (jetstream-only-simple-parallel-3 jetstream-only-cdjs)	7m 37s
build-test-wasmjs	6m 19s
build-test-on-x86-x64-debug	6m 26s
check-tidy	1m 12s
test-on-windows-clang-cl (x86, -m32)	12m 25s
test-on-windows-x86-x64 (x86)	39m 33s
test-on-windows-x86-x64 (x64)	43m 37s
	4h 57m 13s

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