[PECOFF] Support module-definition file.

Module-definition (.def) files are the file containing linker directives,
such as export symbols. Because link.exe supports the same features as command
line options, just as some Linker Script commands overlaps with command line
options, use of module-definition file is not really necessary. It provides
an alternative way to specify some linker options.

This patch implements EXPORTS directive. Other directives will be implemented
in the future.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198925 91177308-0d34-0410-b5e6-96231b3b80d8