Allow gdbremote process to read modules from memory

Summary:
The logic to read modules from memory was added to LoadModuleAtAddress
in the dynamic loader, but not in process gdb remote. This means that when
the remote uses svr4 packets to give library info, libraries only present
on the remote will not be loaded.

This patch therefore involves some code duplication from LoadModuleAtAddress
in the dynamic loader, but removing this would require some amount of code
refactoring.

Reviewers: ADodds, tberghammer, tfiala, deepak2427, ted

Subscribers: tfiala, lldb-commits, sas

Differential Revision: http://reviews.llvm.org/D18531

Change by Francis Ricci <fjricci@fb.com>

This is a cherry-pick of r265418

llvm-svn: 270125