Auto merge of #21028 - asajeffrey:script-dont-trace-parsers, r=Manishearth

Fixed panic due to GC while parsing

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Don't trace the incomplete parsing contexts during GC, as parsing triggers GC while holding a mutable borrow of the context.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #21020

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- [X] These changes do not require tests because fixing a panic

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