COFF: Support creating DLLs.

DLL files are in the same format as executables but they have export tables.
The format of the export table is described in PE/COFF spec section 5.3.

A new class, EdataContents, takes care of creating chunks for export tables.
What we need to do is to parse command line flags for dllexports, and then
instantiate the class to create chunks. For the writer, export table chunks
are opaque data -- it just add chunks to .edata section.

llvm-svn: 239869