draw,llvmpipe: Avoid named struct types on LLVM 3.0 and later.

draw,llvmpipe: Avoid named struct types on LLVM 3.0 and later.

Starting with LLVM 3.0, named structures are meant not for debugging, but
for recursive data types, previously also known as opaque types.

The recursive nature of these types leads to several memory management
difficulties.  Given that we don't actually need recursive types, avoid
them altogether.

This is an attempt to address fdo bugs 41791 and 44466. The issue is
somewhat random so there's no easy way to check how effective this is.