method to get a SymbolBody and into the callers, and kill now dead
includes.

This removes the need to have the SymbolBody definition when we're
defining the inline method and makes it a better inline method. That was
the only reason for a lot of header includes here. Removing these and
using forward declarations actually uncovers a bunch of cross-header
dependencies that I've fixed while I'm here, and will allow me to
introduce some *important* inline code into Chunks.h that requires the
definition of ObjectFile.

No functionality changed at this point.

Differential Revision: http://reviews.llvm.org/D10789

llvm-svn: 240982