Rename EvalParameterPack to EvalPackExpansion

EvalParameterPack was a misleading name. This evaluation represents
a pack expansion expression; the expansion pattern has to contain
one or more parameter packs, but they can be nested inside an
arbitrarily complex expression.

Change-Id: I16d09c349fc6c2b1086e37fd6743f613f02bf5eb
8 files changed