COFF: improve link conformance for import names

Improve the link conformance for the import name embedded into the
import library.  This requires the associated change to the LLVM portion
for the DEF file parser.  The import file generation embeds a different
name based on whether the driver is invoked as "link" or "lib".
Furthermore, the LIBRARY keyword in the DEF file influences the import
name.  The behaviour can be summarised according to the following table:

      | LIBRARY w/ ext |   LIBRARY w/o ext   | no LIBRARY
 -----+----------------+---------------------+------------------
 LINK | {value}        | {value}.{.dll/.exe} | {output name}
  LIB | {value}        | {value}.dll         | {output name}.dll

llvm-svn: 308407