[ATTRIBUTE] Support base vector types of __attribute__((mode)), patch by Alexey Frolov

Base type of attribute((mode)) can actually be a vector type.
The patch is to distinguish between base type and base element type.

This fixes http://llvm.org/PR17453.
Differential Revision: http://reviews.llvm.org/D10058


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240125 91177308-0d34-0410-b5e6-96231b3b80d8