[PECOFF] Support export-only-by-ordinal exports.

If NONAME option is given for an export, that symbol will be exported only by
its ordinal. LLD will not emit the symbol name to the export table.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197371 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Add a check for duplicate export ordinals.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197373 91177308-0d34-0410-b5e6-96231b3b80d8
[PECOFF] Rename ExecutableWriter -> PECOFFWriter.

Because it's now able to emit not only executable but DLL.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197374 91177308-0d34-0410-b5e6-96231b3b80d8
Remove obsolete comment.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@197375 91177308-0d34-0410-b5e6-96231b3b80d8