Switch TableGen to emit calls to ASTRecordReader for AttrPCHRead.

Summary:
This patch changes TableGen-generated code in AttrPCHRead to call functions on
ASTRecordReader, instead of passing separate parameters to ASTReader. This is a
follow-up to r290217.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D28007

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@292868 91177308-0d34-0410-b5e6-96231b3b80d8