c++: Allow mixing GNU/std-style attributes [PR69585]

c++: Allow mixing GNU/std-style attributes [PR69585]

cp_parser_attributes_opt doesn't accept GNU attributes followed by
[[]] attributes and vice versa; only a sequence of attributes of the
same kind.  That causes grief for code like:

  struct __attribute__ ((may_alias)) alignas (2) struct S { };

or

  #define EXPORT __attribute__((visibility("default")))
  struct [[nodiscard]] EXPORT F { };

It doesn't seem to a documented restriction, so this patch fixes the
problem.

However, the patch does not touch the C FE.  The C FE doesn't have
a counterpart to C++'s cp_parser_attributes_opt -- it only has
c_parser_transaction_attributes (which parses both GNU and [[]]
attributes), but that's TM-specific.  The C FE seems to use either
c_parser_gnu_attributes or c_parser_std_attribute_specifier_sequence.
As a consequence, this works:

  [[maybe_unused]] __attribute__((deprecated)) void f2 ();

but this doesn't:

  __attribute__((deprecated)) [[maybe_unused]] void f1 ();

I'm not sure what, if anything, should be done about this.

	PR c++/102399
	PR c++/69585

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_attributes_opt): Accept GNU attributes
	followed by [[]] attributes and vice versa.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/attrib65.C: New test.
	* g++.dg/ext/attrib66.C: New test.
	* g++.dg/ext/attrib67.C: New test.