Add .rgba syntax extension to ext_vector_type types

Summary:
This patch enables .rgba accessors to ext_vector_type types and adds
tests for syntax validation and code generation.

'a' and 'b' can appear either in the point access mode or the numeric
access mode (for indices 10 and 11).  To disambiguate between the two
usages, the accessor type is explicitly passed to relevant methods.

Reviewers: rsmith

Subscribers: Anastasia, bader, srhines, cfe-commits

Differential Revision: http://reviews.llvm.org/D20602

llvm-svn: 276455