[patch] Fix pr9701

[Rafael Espíndola]

The attached patch fixes pr9701. It was marked wontfix, but I could
not find why.

Requiring -Werror=unknown-warning-option is a fairly annoying behavior
difference from gcc.

Before this patch:

$ clang -HHH -c test.c; echo $?
clang: warning: argument unused during compilation: '-HHH'
0

with this patch:

$ clang -HHH -c test.c; echo $?
clang-3.4: error: unknown argument: '-HHH'
1

Cheers,
Rafael
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