Add AST logging

Summary:
Log the AST of the TU associated with LLDB's `expr` command, once a declaration
is completed

Reviewers: shafik

Subscribers: rnkovacs, dkrupp, Szelethus, gamesh411, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D62061

llvm-svn: 361362