Attributes Rewrite

Attributes Rewrite

Convert the internal representation of the Attributes class into a pointer to an
opaque object that's uniqued by and stored in the LLVMContext object. The
Attributes class then becomes a thin wrapper around this opaque
object. Eventually, the internal representation will be expanded to include
attributes that represent code generation options, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165917 91177308-0d34-0410-b5e6-96231b3b80d8