[OpenCL] Fix __builtin_astype for vec3 types.

__builtin_astype does not generate correct LLVM IR for vec3 types. This patch inserts bitcasts to/from vec4 when necessary in addition to generating vector shuffle. Sema and codegen tests are added.

Differential Revision: http://reviews.llvm.org/D20133

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272153 91177308-0d34-0410-b5e6-96231b3b80d8