[FIX] clang6.0 warning caused by Holder<TValue, Tristate> [-Wenum-compare-switch]
[FIX] clang6.0 warning caused by Holder<TValue, Tristate> [-Wenum-compare-switch]

/include/seqan/basic/holder_tristate.h:939:14: warning: comparison of two values with different enumeration types in switch statement ('seqan::Holder<const seqan::StringSet<seqan::Segment<const seqan::String<seqan::SimpleType<unsigned char,
      seqan::Dna5Q_>, seqan::Alloc<void> >, seqan::InfixSegment>, seqan::Dependent<seqan::Tag<seqan::TagInsist_> > >, seqan::Tag<seqan::Tristate_> >::EHolderState' and 'seqan::Holder<seqan::StringSet<seqan::Segment<const seqan::String<seqan::SimpleType<unsigned char,
      seqan::Dna5Q_>, seqan::Alloc<void> >, seqan::InfixSegment>, seqan::Dependent<seqan::Tag<seqan::TagInsist_> > >, seqan::Tag<seqan::Tristate_> >::EHolderState') [-Wenum-compare-switch]
        case Holder<TValue, Tristate>::EMPTY:
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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