c++: Adjust mangling of __alignof__ [PR88115]

c++: Adjust mangling of __alignof__ [PR88115]

r11-4926 made __alignof__ get mangled differently from alignof,
encoding __alignof__ as a vendor extended operator.  But this
mangling is problematic for the reasons mentioned in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88115#c6.

This patch changes our mangling of __alignof__ to instead use the
new "vendor extended expression" syntax that's proposed in
https://github.com/itanium-cxx-abi/cxx-abi/issues/112.  Clang does
the same thing already, so after this patch Clang and GCC agree
about the mangling of __alignof__(type) and __alignof__(expr).

gcc/cp/ChangeLog:

	PR c++/88115
	* mangle.c (write_expression): Adjust the mangling of
	__alignof__.

include/ChangeLog:

	PR c++/88115
	* demangle.h (enum demangle_component_type): Add
	DEMANGLE_COMPONENT_VENDOR_EXPR.

libiberty/ChangeLog:

	PR c++/88115
	* cp-demangle.c (d_dump, d_make_comp, d_expression_1)
	(d_count_templates_scopes): Handle DEMANGLE_COMPONENT_VENDOR_EXPR.
	(d_print_comp_inner): Likewise.
	<case DEMANGLE_COMPONENT_EXTENDED_OPERATOR>: Revert r11-4926
	change.
	<case DEMANGLE_COMPONENT_UNARY>: Likewise.
	* testsuite/demangle-expected: Adjust __alignof__ tests.

gcc/testsuite/ChangeLog:

	PR c++/88115
	* g++.dg/cpp0x/alignof7.C: Adjust expected mangling.