COFF: Create import library files.

On Windows, we have to create a .lib file for each .dll.
When linking against DLLs, the linker doesn't use the DLL files,
but instead read a list of dllexported symbols from corresponding
lib files.

A library file containing descriptors of a DLL is called an
import library file.

lib.exe has a feature to create an import library file from a
module-definition file. In this patch, we create a module-definition
file and pass that to lib.exe.

We eventually want to create an import library file by ourselves
to eliminate dependency to lib.exe. For now, we just use the MSVC
tool.

llvm-svn: 239937