Object: Add support for bigobj

This adds support for reading the "bigobj" variant of COFF produced by
cl's /bigobj and mingw's -mbig-obj.

The most significant difference that bigobj brings is more than 2**16
sections to COFF.

bigobj brings a few interesting differences with it:
- It doesn't have a Characteristics field in the file header.
- It doesn't have a SizeOfOptionalHeader field in the file header (it's
  only used in executable files).
- Auxiliary symbol records have the same width as a symbol table entry.
  Since symbol table entries are bigger, so are auxiliary symbol
  records.

Write support will come soon.

Differential Revision: http://reviews.llvm.org/D5259
[PECOFF] Add support for bigobj

lld shouldn't directly use the COFF header nor should it use raw
coff_symbols.  Instead, query the header properties from the
COFFObjectFile and use COFFSymbolRef to abstractly reference COFF
symbols.

This is just enough to get lld compiling with the changes to
llvm::object.  Bigobj specific testing will come later.

Differential Revision: http://reviews.llvm.org/D5280