[Swiftify] Escape param decl refs (#81550)
Parameters can be named with keywords without escaping, because it's
unambiguous in the grammar that they are parameters. They still need to
escaped when referred to inside the function body however. This escapes
all references to parameters using backticks.
Parameter names are also checked for clashes with the function name - in
such cases the parameter is renamed in the same way as unnamed
parameters.
rdar://151024645
