Now Attributes are divided in three groups

Now Attributes are divided in three groups
- return attributes - inreg, zext and sext
- parameter attributes
- function attributes - nounwind, readonly, readnone, noreturn

Return attributes use 0 as the index.
Function attributes use ~0U as the index.

This patch requires corresponding changes in llvm-gcc and clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56704 91177308-0d34-0410-b5e6-96231b3b80d8