Re-land "COFF: migrate def parser from LLD to LLVM"

This reverts commit r304561 and re-lands r303490 & co.

The fix was to use "SymbolName" when translating LLD's internal export
list to lib/Object's short export struct. The SymbolName reflects the
actual symbol name, which may include fastcall and stdcall mangling bits
not included in the /EXPORT or .def file EXPORTS name:

@@ -434,8 +434,7 @@ std::vector<COFFShortExport> createCOFFShortExportFromConfig() {
   std::vector<COFFShortExport> Exports;
   for (Export &E1 : Config->Exports) {
     COFFShortExport E2;
-    E2.Name = E1.Name;
+    // Use SymbolName, which will have any stdcall or fastcall qualifiers.
+    E2.Name = E1.SymbolName;
     E2.ExtName = E1.ExtName;
     E2.Ordinal = E1.Ordinal;
     E2.Noname = E1.Noname;

llvm-svn: 304573