[PATCH][libcxx] Fix handling of control escapes "\cX" in std::regex

[William Fisher]

This patch fixes an ill-formed comparison when parsing control escapes,
e.g. "\cA\ca". The code will now throw an error_escape exception for
invalid control sequences like "\c:" or "\c".

I've added the test cases to bad_escape.pass.cpp.

Thanks,
-Bill
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