[patch] Support PE/COFF in COFFObjectFile, fix some bugs object file readers

Hi,

The attached patch does a few separate things. I can split them up if you like but it's pretty small.

   - Adds support for sniffing PE/COFF files on win32 (.exe and .dll)
     which are COFF files that have an MS-DOS compatibility stub on
     the front of them.

   - Fixes a bug in the COFFObjectFile's support for the Microsoft COFF
     extension for long symbol names, wherein it was attempting to parse
     the leading '/' in an extended symbol name reference as part of the
     integer offset.

   - Fixes bugs in COFFObjectFile and ELFObjectFile wherein section
     and symbol iterators were being returned with uninitialized bytes;
     the type DataRefImpl is a union between 2 32-bit words (d.a and d.b)
     and a single intptr_t word (p). Only p was being initialized, so in
     32-bit builds the result would be iterators with random upper 32-bit
     words in their DataRefImpls. This caused random failures when
     seeking around in object files.

Let me know if any of this is questionable or you've suggestions on style, as I may submit subsequent bugs along these lines; we're using the LLVM object-file reading facilities in rustc, so will be fixing any bugs we encounter as we go.

Thanks,

-Graydon

pecoff.patch (4.61 KB)

Committed thusly:

[issola:~/sources/llvm] echristo% svn ci
Sending lib/Object/COFFObjectFile.cpp
Sending lib/Object/ELFObjectFile.cpp
Sending lib/Support/Path.cpp
Transmitting file data ...
Committed revision 128799.

-eric