Revert a tiny bit of r148553 which extended LLVM's function attributes

Revert a tiny bit of r148553 which extended LLVM's function attributes
to 64-bits, and added a new attribute in bit #32. Specifically, remove
this new attribute from the enum used in the C API. It's not yet clear
what the best approach is for exposing these new attributes in the
C API, and several different proposals are on the table. Until then, we
can simply not expose this bit in the API at all.

Also, I've reverted a somewhat unrelated change in the same revision
which switched from "1 << 31" to "1U << 31" for the top enum. While "1
<< 31" is technically undefined behavior, implementations DTRT here.
However, MS and -pedantic mode warn about non-'int' type enumerator
values. If folks feel strongly about this I can put the 'U' back in, but
it seemed best to wait for the proper solution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148937 91177308-0d34-0410-b5e6-96231b3b80d8