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