Drop a StringMap.h include, NFC

$ diff -u <(sort thedeps-before.txt) <(sort thedeps-after.txt) \
    | grep '^[-+] ' | sort | uniq -c | sort -nr
    231 -    llvm/include/llvm/ADT/StringMap.h
    171 -    llvm/include/llvm/Support/AllocatorBase.h
    142 -    llvm/include/llvm/Support/PointerLikeTypeTraits.h