Set GenSigCtxt for the argument part of tcSubType
The reason for this change is described in TcUnify Note [Settting the argument context], and Trac #15438. The only effect is on error messages, where it stops GHC reporting an outright falsity (about the type signature for a function) when it finds an errors in a higher-rank situation. The testsuite changes in this patch illustrate the problem.
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