Handle "file not found" error in the superclass's errStr().

If a subclass does not override the member function, the superclass's method
takes care of string conversion of "file not found" error. This is a reasonable
default behavior. Subclasses are still able to override to customize error
messages.

llvm-svn: 191058

lld/include/lld/Driver/InputGraph.h

@@ -226,7 +226,9 @@ public:
   }
 
   /// \brief create an error string for printing purposes
-  virtual std::string errStr(llvm::error_code) {
+  virtual std::string errStr(llvm::error_code errc) {
+    if (errc == llvm::errc::no_such_file_or_directory)
+      return (Twine("Cannot open ") + _path).str();
     llvm_unreachable("not handling errors");
   }