[modules] Start moving the code for encoding AST records out of ASTWriter into

a separate class. The goal is for this class to have a separate lifetime from
the AST writer so that it can meaningfully track pending statement nodes and
context for more compact encoding of various types.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@265195 91177308-0d34-0410-b5e6-96231b3b80d8