First round of extended vector support. Here is an overview...

- added ocu_vector_type attribute, Sema::HandleOCUVectorTypeAttribute(). 
- added new AST node, OCUVectorType, a subclass of VectorType.
- added ASTContext::getOCUVectorType.
- changed ASTContext::convertToVectorType() to ASTContext::getVectorType(). This is 
unrelated to extended vectors, however I was in the vicinity and it was on my todo list.
Added a FIXME to Sema::HandleVectorTypeAttribute to deal with converting complex types.

llvm-svn: 40007