As discussed in IRC with @rsmith, it is slightly not good to keep that in the CastExpr itself:
Given the explicit cast, which is represented in AST as an ExplicitCastExpr + ImplicitCastExpr's,
only the ImplicitCastExpr's will be marked as PartOfExplicitCast, but not the ExplicitCastExpr itself.
Thus, it is only ever true for ImplicitCastExpr's, so we don't need to write/read/dump it for ExplicitCastExpr's.
We don't need to worry that we write the PartOfExplicitCast in PCH after CastExpr::path_iterator,
since the ExprImplicitCastAbbrev is only used when the NumBaseSpecs == 0, i.e. there is no 'path'.