c++: fewer allocator temps [PR105838]

In this PR, initializing the array of std::string to pass to the vector
initializer_list constructor gets very confusing to the optimizers as the
number of elements increases, primarily because of all the std::allocator
temporaries passed to all the string constructors.  Instead of creating one
for each string, let's share an allocator between all the strings; we can do
this safely because we know that std::allocator is stateless and that string
doesn't care about the object identity of its allocator parameter.

	PR c++/105838

gcc/cp/ChangeLog:

	* cp-tree.h (is_std_allocator): Declare.
	* constexpr.cc (is_std_allocator): Split out  from...
	(is_std_allocator_allocate): ...here.
	* init.cc (find_temps_r): New.
	(find_allocator_temp): New.
	(build_vec_init): Use it.

gcc/testsuite/ChangeLog:

	* g++.dg/tree-ssa/allocator-opt1.C: New test.