[readobj] Expand CodeView dumping functionality
This rewrites and expands the existing codeview dumping functionality in
llvm-readobj using techniques similar to those in lib/Object. This defines a
number of new records and enums useful for reading memory mapped codeview
sections in COFF objects.
The dumper is intended as a testing tool for LLVM as it grows more codeview
Differential Revision: http://reviews.llvm.org/D16104