Forward declare special case lists

This avoids pulling in unordered_map from TrigramIndex.h:

$ diff -u thedeps-before.txt thedeps-after.txt | \
        grep '^[-+] ' | sort | uniq -c | sort -nr
    976 -    ../llvm/include/llvm/Support/TrigramIndex.h
    976 -    ../llvm/include/llvm/Support/SpecialCaseList.h
    976 -    ../clang/include/clang/Basic/SanitizerSpecialCaseList.h
    502 -    ../../../PROGRA~2/MIB055~1/2019/PROFES~1/VC/Tools/MSVC/14.24.28314/include/unordered_map
    467 -    ../../../PROGRA~2/MIB055~1/2019/PROFES~1/VC/Tools/MSVC/14.24.28314/include/xhash
    467 -    ../../../PROGRA~2/MIB055~1/2019/PROFES~1/VC/Tools/MSVC/14.24.28314/include/xbit_ops.h
    212 -    ../llvm/include/llvm/ADT/StringSet.h