pdbdump: print out COFF section headers.

Unlike other sections that can grow to any size, the COFF section header
stream has maximum length because each record is fixed size and the COFF
file format limits the maximum number of sections. So I decided to not
create a specific stream class for it. Instead, I added a member function
to DbiStream class which returns a vector of COFF headers.

Differential Revision: http://reviews.llvm.org/D20717

llvm-svn: 271557