COFF: Create short import files instead of using lib.exe.

lib.exe has a feature to create import library files (which contain
short import files) from module-definition files. Previously, we were
using that feature, but it turned out that the feature is not complete
for us.

There seems no way to specify "Import Types" in module-definition file.
lib.exe always adds "_" to given symbols and specify IMPORT_NAME_UNDECORATE.
We need more fine-grainded control on that value.

This patch teaches LLD to create short import files itself.
We are still using lib.exe, but the use of the tool is limited to create
empty import library files. We then create short import files and add them
to the empty files as new members.

This patch does not intend to change the functionality. LLD produces
the same import libraries as before. I'll make another change to create
different import libraries in a follow-up patch.

llvm-svn: 246292