Bug 1389851 - build: Enable clang's -Wunreachable-code-return warnings. r=glandium

MozReview-Commit-ID: 8Zl0mbzcFBm

--HG--
extra : rebase_source : 36c92b50fac9e1f7991d977e1c878b4945e97528

build/moz.configure/warnings.configure

@@ -42,6 +42,7 @@ add_gcc_warning('-Wtype-limits')
 
 # catches some dead code
 add_gcc_warning('-Wunreachable-code')
+check_and_add_gcc_warning('-Wunreachable-code-return')
 
 # catches treating string literals as non-const
 add_gcc_warning('-Wwrite-strings', cxx_compiler)