Add vector type mangling (as discussed on the IA-64 reflector in 2009)

[MikeHerrick]

This adds Dv<number>_<type> mangling for vector types (as discussed on the old IA-64 ABI reflector in November 2009).

[zygoloid]

If a vendor supports multiple flavours of vector type at once, which type should receive this mangling? Clang currently uses the Dv mangling for attribute(vector_size), attribute(ext_vector_type), and AltiVec vectors (spelled with the vector or __vector keyword), but has a different mangling for ARM NEON vectors. The reuse of Dv for multiple distinct kinds of vector can lead to mangling collisions in Clang currently.

[khuey]

It would nice if this were made official at some point. libiberty supports different syntax than LLVM (for dimension expressions it does Dv_<expression> rather than Dv<expression>) but as far as I can tell gcc doesn't actually emit vector types with dimension expressions yet.