Object: handle extensions properly in def files

When given an extension as part of the `library` directive in a def
file, the extension is preserved/honoured by link/lib.  Behave similarly
when parsing the def file.  This requires checking if a native extension
is provided as a keyword parameter.  If no extension is present, append
a standard `.dll` or `.exe` extension.

This is best tested via lld, and I will add tests there as a follow up.

llvm-svn: 308383

llvm/lib/Object/COFFModuleDefinition.cpp

@@ -22,6 +22,7 @@
 #include "llvm/Object/COFFImportFile.h"
 #include "llvm/Object/Error.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm::COFF;
@@ -188,9 +189,8 @@ private:
       if (Error Err = parseName(&Name, &Info.ImageBase))
         return Err;
       // Append the appropriate file extension if not already present.
-      StringRef Ext = IsDll ? ".dll" : ".exe";
-      if (!StringRef(Name).endswith_lower(Ext))
-        Name += Ext;
+      if (!sys::path::has_extension(Name))
+        Name += IsDll ? ".dll" : ".exe";
 
       // Set the output file, but don't override /out if it was already passed.
       if (Info.OutputFile.empty())