[cfe] Introduce StructuralParameters

This CL introduces StructuralParameter and StructuralParameterType
classes. They are intended to replace TypeParameter and
TypeParameterType respectively where those were used as type
parameters defined by FunctionTypes. Previously, type parameters of
FunctionTypes were represented by TypeParameter objects with the
‘parent’ field set to null. By introducing StructuralParameter and
StructuralParameterType this CL unambiguously separates the two
notions of type parameters.

TEST=existing

Change-Id: Ida3feb7ad96a7b2acef55840eacba9e36bf2a3e7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/312264
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>