[libc++] Initialized variables for compilers that do not allow this in constexpr.

[mpark]

Summary

In C++20, variables in constexpr contexts are not required to be initialized.
But there are released compilers that do not implement this under -std=c++2a.

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy.pass.cpp

 #include "test_macros.h"
 #include "test_iterators.h"
 
 template <class InIter, class OutIter>
 TEST_CONSTEXPR_CXX20 void
 test_copy()
 {
     const unsigned N = 1000;
-    int ia[N];
+    int ia[N] = {};
     for (unsigned i = 0; i < N; ++i)
         ia[i] = i;
     int ib[N] = {0};
 
     OutIter r = std::copy(InIter(ia), InIter(ia+N), OutIter(ib));
     assert(base(r) == ib+N);
     for (unsigned i = 0; i < N; ++i)
         assert(ia[i] == ib[i]);

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_backward.pass.cpp

 #include "test_iterators.h"
 #include "user_defined_integral.h"
 
 template <class InIter, class OutIter>
 TEST_CONSTEXPR_CXX20 void
 test_copy_backward()
 {
     const unsigned N = 1000;
-    int ia[N];
+    int ia[N] = {};
     for (unsigned i = 0; i < N; ++i)
         ia[i] = i;
     int ib[N] = {0};
 
     OutIter r = std::copy_backward(InIter(ia), InIter(ia+N), OutIter(ib+N));
     assert(base(r) == ib);
     for (unsigned i = 0; i < N; ++i)
         assert(ia[i] == ib[i]);

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_if.pass.cpp

struct Pred
     TEST_CONSTEXPR_CXX14 bool operator()(int i) {return i % 3 == 0;}
 };
 
 template <class InIter, class OutIter>
 TEST_CONSTEXPR_CXX20 void
 test_copy_if()
 {
     const unsigned N = 1000;
-    int ia[N];
+    int ia[N] = {};
     for (unsigned i = 0; i < N; ++i)
         ia[i] = i;
     int ib[N] = {0};
 
     OutIter r = std::copy_if(InIter(ia), InIter(ia+N), OutIter(ib), Pred());
     assert(base(r) == ib+N/3+1);
     for (unsigned i = 0; i < N/3+1; ++i)
         assert(ib[i] % 3 == 0);

libcxx/test/std/algorithms/alg.modifying.operations/alg.copy/copy_n.pass.cpp

 
 typedef UserDefinedIntegral<unsigned> UDI;
 
 template <class InIter, class OutIter>
 TEST_CONSTEXPR_CXX20 void
 test_copy_n()
 {
     const unsigned N = 1000;
-    int ia[N];
+    int ia[N] = {};
     for (unsigned i = 0; i < N; ++i)
         ia[i] = i;
     int ib[N] = {0};
 
     OutIter r = std::copy_n(InIter(ia), UDI(N/2), OutIter(ib));
     assert(base(r) == ib+N/2);
     for (unsigned i = 0; i < N/2; ++i)
         assert(ia[i] == ib[i]);