llvm; Refactor ExecutionMode handling #10057

Job	Run time
build (3.8, x64, ubuntu)	1h 25m 32s
build (3.8, x64, windows)	1h 25m 23s
build (3.11, x64, ubuntu)	1h 9m 0s
build (3.11, x64, macos)	30m 25s
build (3.11, x64, windows)	1h 27m 48s
build (3.12, x64, ubuntu)	1h 10m 45s
build (3.12, x64, macos)	29m 23s
build (3.12, x64, windows)	1h 25m 48s
build (3.9, macos, --cov=psyneulink)	56m 5s
build (3.9, ubuntu, --forked -m "not llvm")	34m 15s
build (3.9, x86, windows)	1h 28m 33s
build (3.10, ubuntu, --fp-precision=fp32)	1h 14m 20s
build (3.10, macos, -m benchmark --benchmark-enable --benchmark-only --benchmark-min-rounds=2 --b...	26m 58s
build (3.8, macos, min)	34m 46s
	14h 19m 1s

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