llvm-objdump: Dump COFF import table if -private-headers option is given.

This is a patch to add capability to llvm-objdump to dump COFF Import Table
entries, so that we can write tests for LLD checking Import Table contents.

llvm-objdump did not print anything but just file name if the format is COFF
and -private-headers option is given. This is a patch adds capability for
dumping DLL Import Table, which is specific to the COFF format.

In this patch I defined a new iterator to iterate over import table entries.
Also added a few functions to COFFObjectFile.cpp to access fields of the entry.

Differential Revision: http://llvm-reviews.chandlerc.com/D1719