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