Wrap `JS_FORGET_STRING_LINEARNESS` #439

Job	Run time
mac (--features debugmozjs)	19m 30s
mac	16m 51s
linux (--features debugmozjs)	15m 43s
linux	7m 58s
windows (--features debugmozjs)	20m 25s
windows (--features debugmozjs, aarch64-uwp-windows-msvc)	11m 19s
windows (--features debugmozjs, x86_64-uwp-windows-msvc)	14m 16s
windows	14m 25s
windows (aarch64-uwp-windows-msvc)	10m 21s
windows (x86_64-uwp-windows-msvc)	13m 51s
Result	1s
	2h 24m 40s

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