attribs: Don't canonicalize lookup_scoped_attribute_spec argument [PR113674]

The C and C++ FEs when parsing attributes already canonicalize them
(i.e. if they start with __ and end with __ substrings, we remove those).
lookup_attribute already verifies in gcc_assert that the first character
of name is not an underscore, and even lookup_scoped_attribute_spec doesn't
attempt to canonicalize the namespace it is passed.  But for some historic
reason it was canonicalizing the name argument, which misbehaves when
an attribute starts with ____ and ends with ____.
I believe it is just wrong to try to canonicalize
lookup_scope_attribute_spec name attribute, it should have been
canonicalized already, in other spots where it is called it is already
canonicalized before.

2024-02-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/113674
	* attribs.cc (extract_attribute_substring): Remove.
	(lookup_scoped_attribute_spec): Don't call it.

	* c-c++-common/Wattributes-3.c: New test.