[windows] Provide a symlink alternative for Windows.

Python 2.7 in Windows do not have os.symlink. Provide one by using Win32
API, and try to match the behaviour that seems to be implied by the Unix
implementation (meaning, the destination is removed before the symlink
is created).

Author: drodriguez

utils/update_checkout/update_checkout/update_checkout.py

@@ -399,6 +399,28 @@ def skip_list_for_platform(config):
     return skip_list
 
 
+# Python 2.7 in Windows doesn't support os.symlink
+os_symlink = getattr(os, "symlink", None)
+if callable(os_symlink):
+    pass
+else:
+    def symlink_ms(source, link_name):
+        source = os.path.normpath(source)
+        link_name = os.path.normpath(link_name)
+        if os.path.isdir(link_name):
+            os.rmdir(link_name)
+        elif os.exists(link_name):
+            os.remove(link_name)
+        import ctypes
+        csl = ctypes.windll.kernel32.CreateSymbolicLinkW
+        csl.argtypes = (ctypes.c_wchar_p, ctypes.c_wchar_p, ctypes.c_uint32)
+        csl.restype = ctypes.c_ubyte
+        flags = 1 if os.path.isdir(source) else 0
+        if csl(link_name, source, flags) == 0:
+            raise ctypes.WinError()
+    os.symlink = symlink_ms
+
+
 def symlink_llvm_monorepo(args):
     print("Create symlink for LLVM Project")
     llvm_projects = ['clang',