Change filesystem lib to recognize paths started with \ as root

[zturner]

Summary

Previously a path of the form

\foo\bar

would not be recognized as a root path on Windows. This patch allows paths of this nature to be considered absolute paths.

[zturner]

Abandoning this as it's obviously wrong. Occurred to me later on that even a path that starts with \ in windows is still a relative path, it's just relative to the current drive.

lib/Support/Path.cpp

#endif
 
   return StringRef();
 }
 
 const StringRef root_name(StringRef path) {
   const_iterator b = begin(path),
                  e = end(path);
   if (b != e) {
-    bool has_net = b->size() > 2 && is_separator((*b)[0]) && (*b)[1] == (*b)[0];
+    bool has_sep = is_separator((*b)[0]);
     bool has_drive =
 #ifdef LLVM_ON_WIN32
       b->endswith(":");
 #else
       false;
 #endif
 
-    if (has_net || has_drive) {
+    if (has_sep || has_drive) {
       // just {C:,//net}, return the first component.
       return *b;
     }
   }
 
   // No path or no name.
   return StringRef();
 }