Bug 1453990 - Disable -Werror on -Wreturn-std-move. We have a false positive with nsAutoCString r=froydnj

See: https://bugs.llvm.org/show_bug.cgi?id=37249
MozReview-Commit-ID: VFvj33eeOm

--HG--
extra : rebase_source : 75208b2cfd1d3302ec0da2af3b24ceb36a4bdd8b

build/moz.configure/warnings.configure

@@ -102,6 +102,10 @@ check_and_add_gcc_warning('-Wno-error=free-nonheap-object')
 # Would be a pain to fix all occurrences, for very little gain
 check_and_add_gcc_warning('-Wno-error=multistatement-macros')
 
+# Disable the -Werror for return-std-move because of a false positive
+# on nsTAutoStringN: https://bugs.llvm.org/show_bug.cgi?id=37249
+check_and_add_gcc_warning('-Wno-error=return-std-move')
+
 # catches format/argument mismatches with printf
 c_format_warning, cxx_format_warning = check_and_add_gcc_warning(
     '-Wformat', when=depends(target)(lambda t: t.kernel != 'WINNT'))