[libcxx] Avoid spurious construction of valarray elements

Summary:
Currently libc++ implements some operations on valarray by using the
resize method. This method has a parameter with a default value.
Because of this, valarray may spuriously construct and destruct
objects of valarray's element type.
    
This patch fixes this issue and adds corresponding test cases.


Reviewers: EricWF, mclow.lists

Reviewed By: mclow.lists

Subscribers: rogfer01, cfe-commits

Differential Revision: https://reviews.llvm.org/D41992

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324596 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/numerics/numarray/template.valarray/valarray.cons/size.pass.cpp

@@ -16,6 +16,15 @@
 #include <valarray>
 #include <cassert>
 
+struct S {
+    S() : x(1) {}
+    ~S() { ++cnt_dtor; }
+    int x;
+    static size_t cnt_dtor;
+};
+
+size_t S::cnt_dtor = 0;
+
 int main()
 {
     {
@@ -36,4 +45,11 @@ int main()
         for (int i = 0; i < 100; ++i)
             assert(v[i].size() == 0);
     }
+    {
+        std::valarray<S> v(100);
+        assert(v.size() == 100);
+        for (int i = 0; i < 100; ++i)
+            assert(v[i].x == 1);
+    }
+    assert(S::cnt_dtor == 100);
 }