c-family: add attribute flag_enum [PR81665]

c-family: add attribute flag_enum [PR81665]

Several PRs complain about -Wswitch warning about a case for a bitwise
combination of enumerators.  Clang has an attribute flag_enum to prevent
this; let's adopt that approach as well.

This also recognizes the attribute as [[clang::flag_enum]], introducing
handling of the clang attribute namespace.

	PR c++/46457
	PR c++/81665

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_flag_enum_attribute): New.
	(c_common_gnu_attributes): Add it.
	(c_common_clang_attributes, c_common_clang_attribute_table): New.
	* c-common.h: Declare c_common_clang_attribute_table.
	* c-warn.cc (c_do_switch_warnings): Handle flag_enum.

gcc/c/ChangeLog:

	* c-objc-common.h (c_objc_attribute_table): Add
	c_common_clang_attribute_table.

gcc/cp/ChangeLog:

	* cp-objcp-common.h (cp_objcp_attribute_table): Add
	c_common_clang_attribute_table.

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-flag-enum-1.c: New test.

gcc/ChangeLog:

	* doc/extend.texi: Document flag_enum attribute.
	* doc/invoke.texi: Mention flag_enum in -Wswitch.

libstdc++-v3/ChangeLog:

	* include/bits/regex_constants.h: Use flag_enum.