[PATCH] [libcxx] Fix match_results for alternatives

[Marshall Clow]

================
Comment at: test/std/re/re.results/re.results.acc/index.pass.cpp:26
@@ -25,3 +25,3 @@
 
     assert(m[0].first == s+2);
     assert(m[0].second == s+9);
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Ok, then I'm really confused.

The code for `match_results::operator[]` is:
     return __n < __matches_.size() ? __matches_[__n] : __unmatched_;

So how does the change in `<regex>` affect this result?

http://reviews.llvm.org/D7111

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