Add a ProcessStructReader utility class to LLDB

The purpose of a ProcessStructReader is to allow intelligent reading of data from the underlying process
Traditionally, the way this has been handled is to have a load_address and shuffle it around, and use Process::ReadMemory()-kind APIs

With a ProcessStructReader one can define a clang type that matches exactly the definition of the thing they are trying to ingest from the inferior process, and then just point LLDB at where the data is
Since this work is done in terms of clang types, one can honor packed/aligned attributes, sizes of types on the inferior architecture, and similar tricky caveats without having to hardcode them

llvm-svn: 217644