[patch] std::regex: Implement LWG DR 2329 and DR 2332

[Jonathan Wakely]

Two defect reports that protect against dangerous uses of std::regex
involving dangling references to temporaries:
http://cplusplus.github.io/LWG/lwg-defects.html#2329
http://cplusplus.github.io/LWG/lwg-defects.html#2332

Tested x86_64-linux & powerpc64-linux, committed to trunk.
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