[COFF] Add SymbolName as a distinct field in COFFImportFile

The previous Name and ExtName aren't enough to convey all the nuances
between weak aliases and stdcall decorated function names.

A test for this will be added in LLD.

Differential Revision: https://reviews.llvm.org/D36544

llvm-svn: 310988

llvm/lib/Object/COFFImportFile.cpp

@@ -590,7 +590,7 @@ std::error_code writeImportLibrary(StringRef ImportName, StringRef Path,
     if (E.Constant)
       ImportType = IMPORT_CONST;
 
-    StringRef SymbolName = E.isWeak() ? E.ExtName : E.Name;
+    StringRef SymbolName = E.SymbolName.empty() ? E.Name : E.SymbolName;
     ImportNameType NameType = getNameType(SymbolName, E.Name, Machine);
     Expected<std::string> Name = E.ExtName.empty()
                                      ? SymbolName