[wwwdocs] Add some C++ bits to porting_to

[Marek Polacek]

I spotted these two issues in real-world code when doing Fedora mass
rebuild.  It's C++ and so I don't really know what I'm talking about;
Jon, could you please look at this?

Thanks,

Index: porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
retrieving revision 1.3
diff -u -r1.3 porting_to.html
--- porting_to.html     2 Feb 2016 20:34:14 -0000       1.3
+++ porting_to.html     4 Feb 2016 12:33:19 -0000
@@ -107,6 +107,60 @@
   bool valid(std::ostream& os) { return (bool)os; }
 </code></pre>
 
+<h4>Lvalue required as left operand of assignment with complex numbers</h4>
+
+<p>
+Since C++11 (as per DR#387) the member functions <code>real()</code> and 
+<code>imag()</code> of <code>std::complex</code> can no longer be used as
+lvalues, thus the following code is rejected:
+</p>
+<pre><code>
+  std::complex<double> f;
+  f.real () = val;
+</code></pre>
+
+<p>
+To assign <code>val</code> to the real component of <code>f</code>, the
+following should be used instead:
+</p>
+<pre><code>
+  std::complex<double> f;
+  f.real (val);
+</code></pre>
+
+<h4>Destructors are <code>noexcept</code> by default</h4>
+
+<p>
+In C++11, destructors are implicitly declared as <code>noexcept</code>.  In
+practice this means that the following program behaves differently in C++11
+than in C++03:
+</p>
+<pre><code>
+  #include &lt;stdexcept&gt;
+  struct S
+  {
+    ~S() { throw std::runtime_error ("oops"); }
+  };
+  int
+  main (void)
+  {
+    try { S s; }
+    catch (...) {
+      return 42;
+    }
+  }
+</code></pre>
+
+<p>
+While in C++03 this program returns 42, in C++11 it terminates with a call to
+<code>std::terminate</code>.  A new warning, <code>-Wterminate</code>, will
+warn about such code.  It is possible to restore the old behavior when
+defining the destructor like this:
+</p>
+<pre><code>
+    ~S() noexcept(false) { throw std::runtime_error ("oops"); }
+</code></pre>
+
 <h3>Header dependency changes</h3>
 
 <p>

        Marek