Remove llvm::Error include form Diagnostic.h

Saves ~400 related LLVM ADT. llvm/ADT/Error.h takes 90ms to parse.

$ diff -u <(sort thedeps-before.txt) <(sort thedeps-after.txt) \
    | grep '^[-+] ' | sort | uniq -c | sort -nr
    403 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Error.h
    403 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm-c/Error.h
    397 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Format.h
    397 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/Debug.h
    377 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/ADT/StringExtras.h
    158 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm-c/ExternC.h
    138 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/ErrorOr.h
     13 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/Support/raw_ostream.h
     13 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/ADT/SmallString.h
      5 -    /usr/local/google/home/rnk/llvm-project/llvm/include/llvm/ADT/Twine.h