Fix broken handling of whitespace in GenericObject.

This actually makes the code matches its comment, i.e, whitespaces are now
normalized the same way XRefRole does.

Without the fix, when creating a directive/role via add_object_type, the
following testcase fails with "WARNING: 'baz' reference target not found:
foo bar".

    :baz:`foo bar`
    .. baz:: foo bar

The current workaround is to explicitly mention the broken target, e.g.,

    :baz:`foo bar <foobar>`
    .. baz:: foo bar

sphinx/domains/std.py

@@ -62,7 +62,7 @@ class GenericObject(ObjectDescription):
             signode.clear()
             signode += addnodes.desc_name(sig, sig)
             # normalize whitespace like XRefRole does
-            name = ws_re.sub('', sig)
+            name = ws_re.sub(' ', sig)
         return name
 
     def add_target_and_index(self, name: str, sig: str, signode: desc_signature) -> None: