[PECOFF] Refactor module-defintion file parser.

Refactor the parser so that the parser can return arbitrary type of parse
result other than a vector of ExportDesc. Parsers for non-EXPORTS directives
will be implemented in different patches. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@198993 91177308-0d34-0410-b5e6-96231b3b80d8