[IR] Add a target extension type to LLVM.

Target-extension types represent types that need to be preserved through
optimization, but otherwise are not introspectable by target-independent
optimizations. This patch doesn't add any uses of these types by an existing
backend, it only provides basic infrastructure such that these types would work
correctly.

Reviewed By: nikic, barannikov88

Differential Revision: https://reviews.llvm.org/D135202