Generate docs/AttributeReference.rst

$ bin/clang-tblgen -gen-attr-docs -I../cfe.src/include \
    ../cfe.src/include/clang/Basic/Attr.td \
    -o ../cfe.src/docs/AttributeReference.rst


git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_60@321744 91177308-0d34-0410-b5e6-96231b3b80d8