Code model check doesn't work correctly when run with `autorest`

[tjprescott]

Running autorest.swift via the autorest --use command will fail to produce before.json and after.json files when there's a code model discrepancy. Here's the log output in such case:

AutorestSwift.default (WARN) Discrepancies found in round-tripped code model. Error saving files: Error 
Domain=NSCocoaErrorDomain Code=4 "The folder “before.json” doesn’t exist." UserInfo=
{NSFilePath=/var/folders/ss/8mw44zjj7l11pqgn2xwtcl100000gn/T/D07A96CA-AF5F-45CD-8EA9-
47C7AC34B69B/before.json, NSUserStringVariant=Folder, NSUnderlyingError=0x7fd7cfc2e870 {Error 
Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}}