[Stats] Only use input filename, not mangled path, in stats file name.

This was causing cases of very long input pathnames to be mangled into
stats filenames greater than 255 characters long, which in turn meant
stats files were not being written in some cases.

lib/Basic/Statistic.cpp

@@ -100,6 +100,8 @@ auxName(StringRef ModuleName,
   if (InputName.empty()) {
     InputName = "all";
   }
+  // Dispose of path prefix, which might make composite name too long.
+  InputName = path::filename(InputName);
   if (OptType.empty()) {
     OptType = "Onone";
   }