Move ATTRIBUTE_FOO macros to LLVM_ATTRIBUTE_FOO

I’d like to rename all of our macros to have LLVM_ or CLANG_ as a prefix (not sure if there are any where the latter is more appropriate, but I may find them).

As LLVM and Clang are more and more widely used as libraries, these generic names have a high likelihood of collision with existing macros: http://google.com/codesearch?as_q=ATTRIBUTE_UNUSED

I’m adding the new macros now, and if I don’t hear any objections, I’ll switch every use over tomorrow. I’ll then leave the old names for a few days, maybe a week, and then fix any new uses and remove them. Comments welcome. =]