COFF: Support module-definition files.

Module-definition files (.def files) are yet another way to
specify parameters to the linker. You can write a list of dllexported
symbols in module-definition files instead of using /export command
line option. It also supports a few more directives.

The parser code is taken from lib/Driver/WinLinkModuleDef.cpp
with the following modifications.

 - variable names are updated to comply with the LLVM coding style.
 - Instead of returning parsing results as "directive" objects,
   it updates Config object directly.

llvm-svn: 239929