Make `assumeIsolated` work with SerialExecutors that are backed by EventLoops #402

Job	Run time
Soundness / API breakage check	2m 14s
Soundness / Documentation check	2m 6s
Soundness / Unacceptable language check	3s
Soundness / License headers check	10s
Soundness / Broken symlinks check	5s
Soundness / Format check	57s
Soundness / Shell check	7s
Integration Tests / Linux (5.8)	12m 50s
Integration Tests / Linux (5.9)	14m 25s
Integration Tests / Linux (5.10)	14m 2s
Integration Tests / Linux (nightly-6.0)	16m 19s
Integration Tests / Linux (nightly-main)	17m 1s
Cxx interop / Cxx interop / Linux (5.8)	1m 7s
Cxx interop / Cxx interop / Linux (5.9)	1m 14s
Cxx interop / Cxx interop / Linux (5.10)	1m 8s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 42s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 42s
Benchmarks / Benchmarks / Linux (5.8)	3m 10s
Benchmarks / Benchmarks / Linux (5.9)	5m 13s
Benchmarks / Benchmarks / Linux (5.10)	5m 6s
Benchmarks / Benchmarks / Linux (nightly-6.0)	5m 57s
Benchmarks / Benchmarks / Linux (nightly-main)	6m 3s
Unit tests / Unit tests / Linux (5.8)	3m 8s
Unit tests / Unit tests / Linux (5.9)	1m 45s
Unit tests / Unit tests / Linux (5.10)	1m 50s
Unit tests / Unit tests / Linux (nightly-6.0)	4m 17s
Unit tests / Unit tests / Linux (nightly-main)	4m 9s
Swift 6 Language Mode / Swift 6 language mode	50s
	2h 8m 40s

Provide feedback