[compiler] FunctionExpression context locations point to first reference (#33512)

This has always been awkward: `FunctionExpression.context` places have
locations set to the declaration of the identifier, whereas other
references have locations pointing to the reference itself. Here, we
update context operands to have their location point to the first
reference of that variable within the function.

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[//]: # (BEGIN SAPLING FOOTER)
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* #33571
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* __->__ #33512
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