OpenCL vector types

[dkolsen-pgi]

Implement OpenCL vector types, a.k.a. extended vector types, in ClangIR.

Issue #284 implemented GNU vector types. Other issues are being created for other kinds of vector types. See https://clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectors

[jopperm]

@dkolsen-pgi, may I assign this to @seven-mile to tackle as part of his GSoC project?

[dkolsen-pgi]

Assigning this to @seven-mile is fine.