Fix gcc warnings from having only one of assignment/copy

For example:
warning: implicitly-declared
‘constexpr Complex& Complex::operator=(const Complex&)’
is deprecated [-Wdeprecated-copy]

Pick-to: 6.1
Change-Id: I7598e821acb7cb7bf17776d693af62778185afc5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/concurrent/qtconcurrentmap/tst_qtconcurrentmap.cpp

@@ -1618,6 +1618,8 @@ public:
     { currentInstanceCount.fetchAndAddRelaxed(-1);}
     inline InstanceCounter(const InstanceCounter &)
     { currentInstanceCount.fetchAndAddRelaxed(1); updatePeak(); }
+    constexpr InstanceCounter &operator=(const InstanceCounter &) noexcept
+    { return *this; }
 
     void updatePeak()
     {

tests/auto/corelib/tools/containerapisymmetry/tst_containerapisymmetry.cpp

@@ -106,6 +106,8 @@ struct Complex
     {
         --instanceCount;
     }
+    constexpr Complex &operator=(const Complex &o) noexcept
+    { i = o.i; return *this; }
 
     int i;
     static int instanceCount;

tests/auto/corelib/tools/qcache/tst_qcache.cpp

@@ -359,6 +359,7 @@ struct KeyType
     int foo;
 
     KeyType(int x) : foo(x) {}
+    constexpr KeyType(const KeyType &o) noexcept : foo(o.foo) {}
 
 private:
     KeyType &operator=(const KeyType &);

tests/auto/corelib/tools/qhash/tst_qhash.cpp

@@ -97,6 +97,8 @@ struct Foo {
     Foo():c(count) { ++count; }
     Foo(const Foo& o):c(o.c) { ++count; }
     ~Foo() { --count; }
+    constexpr Foo &operator=(const Foo &o) noexcept { c = o.c; return *this; }
+
     int c;
     int data[8];
 };