[SPIR-V] Support TargetExtType for SPIR-V builtin types

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.

Differential Revision: https://reviews.llvm.org/D144494