Bill 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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@186335 91177308-0d34-0410-b5e6-96231b3b80d8