This patch adds support for TargetExtType/target(...) representing SPIR-V builtin types. After D135202, target(...) is the preferred way for representing SPIR-V builtin types in LLVM IR and the only working in the opaque pointer mode.
In order to maintain compatibility with LLVM IR generated by older versions of Clang and LLVM/SPIR-V Translator, pointers-to-opaque-structs denoting SPIR-V/OpenCL builtin types will be translated to equivalent SPIR-V target extension types. This translation is only available in the typed pointer mode (-opaque-pointers=0).
The relevant LIT tests with SPIR-V builtins were converted to use the new target(...) notation.