Remove unused Endian.h includes, NFC

Mainly avoids including Host.h everywhere:

$ diff -u <(sort thedeps-before.txt) <(sort thedeps-after.txt) \
    | grep '^[-+] ' | sort | uniq -c | sort -nr
   3141 - /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Host.h