[AST] Sink 'part of explicit cast' down into ImplicitCastExpr

Summary:
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'.

Reviewers: rsmith, rjmccall, erichkeane, aaron.ballman

Reviewed By: rsmith, erichkeane

Subscribers: vsk, cfe-commits, rsmith

Tags: #clang

Differential Revision: https://reviews.llvm.org/D49838

llvm-svn: 338108