c++: Further address_compare fixes [PR89074]

c++: Further address_compare fixes [PR89074]

This patch introduces folding_cxx_constexpr, folding_initializer is used
for both C and C++ initializer/constant expression folding and enables more
optimizations over what we do normally at runtime, while folding_cxx_constexpr
is used only during C++ constant expression folding and disables some optimizations.

The patch improves STRING_CST vs. STRING_CST folding, for folding_initializer
FUNCTION_DECL vs. FUNCTION_DECL folding, disables some optimizations like
is_global_var != is_global_var or STRING_CST vs. DECL_P for folding_cxx_constexpr
etc.

2022-02-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/89074
	PR c++/104033
	* fold-const.h (folding_initializer): Adjust comment.
	(folding_cxx_constexpr): Declare.
	* fold-const.cc (folding_initializer): Adjust comment.
	(folding_cxx_constexpr): New variable.
	(address_compare): Restrict the decl vs. STRING_CST
	or vice versa or STRING_CST vs. STRING_CST or
	is_global_var != is_global_var optimizations to !folding_cxx_constexpr.
	Punt for FUNCTION_DECLs with non-zero offsets.  If folding_initializer,
	assume non-aliased functions have non-zero size and have different
	addresses.  For folding_cxx_constexpr, punt on comparisons of start
	of some object and end of another one, regardless whether it is a decl
	or string literal.  Also punt for folding_cxx_constexpr on
	STRING_CST vs. STRING_CST comparisons if the two literals could be
	overlapping.

	* constexpr.cc (cxx_eval_binary_expression): Temporarily set
	folding_cxx_constexpr.

	* g++.dg/cpp1y/constexpr-89074-3.C: New test.