Fix missing SymCo in pushCoercionIntoLambda

sheafrequested to merge
sheaf/ghc:pushCoercionIntoLambda into master

There was a missing SymCo in pushCoercionIntoLambda. Currently this codepath is only used with rewrite rules, so this bug managed to slip by, but trying to use pushCoercionIntoLambda in other contexts revealed the bug.

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