[SPIR-V] Add SPIR-V triple and clang target info.

Add new triple and target info for ‘spirv32’ and ‘spirv64’ and,
thus, enabling clang (LLVM IR) code emission to SPIR-V target.

The target for SPIR-V is mostly reused from SPIR by derivation
from a common base class since IR output for SPIR-V is mostly
the same as SPIR. Some refactoring are made accordingly.

Added and updated tests for parts that are different between
SPIR and SPIR-V.

Patch by linjamaki (Henry Linjamäki)!

Differential Revision: https://reviews.llvm.org/D109144