Bug 1406618 - Enable gcc -Wduplicated-cond warnings. r=glandium

Warn about duplicated conditions in if-else-if chains, which are unreachable code and likely copy/paste bugs. The -Wduplicated-cond flag is available in gcc 6 and later.

  int example(int a)
  {
    if (a == 0)
      a = 42;
    else if (a == 0) // -Wduplicated-cond warning!
      a = 43;
    return a;
  }

MozReview-Commit-ID: F9hqxMCct74

--HG--
extra : rebase_source : 71168f79ce8f03f650167cdedbbd2e5802846eab

build/moz.configure/warnings.configure

@@ -65,6 +65,9 @@ check_and_add_gcc_warning('-Wc++1z-compat', cxx_compiler)
 # catches possible misuse of the comma operator
 check_and_add_gcc_warning('-Wcomma', cxx_compiler)
 
+# catches duplicated conditions in if-else-if chains
+check_and_add_gcc_warning('-Wduplicated-cond')
+
 # catches unintentional switch case fallthroughs
 check_and_add_gcc_warning('-Wimplicit-fallthrough', cxx_compiler)