[mlir][llvm] Add LLVM TargetExtType

Add support for the `llvm::TargetExtType` to the MLIR LLVM dialect.

Target extension types were introduced to represent target-specific types, which are opaque to the compiler and optimizations.

The patch also enforces some of the constraints defined for the target extension type in the LLVM language reference manual.

Signed-off-by: Lukas Sommer <lukas.sommer@codeplay.com>

Reviewed By: ftynse, gysit, Dinistro

Differential Revision: https://reviews.llvm.org/D151446