switch a bunch of ASTReader to use the new BitstreamCursor::advance* methods,
which hide a bunch of private details of the cursor from clients and simplify
their code.  More to come.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172922 91177308-0d34-0410-b5e6-96231b3b80d8