[mlir] LLVM dialect: Generate conversions between EnumAttrCase and LLVM API

Summary:
MLIR materializes various enumeration-based LLVM IR operands as enumeration
attributes using ODS. This requires bidirectional conversion between different
but very similar enums, currently hardcoded. Extend the ODS modeling of
LLVM-specific enumeration attributes to include the name of the corresponding
enum in the LLVM C++ API as well as the names of specific enumerants. Use this
new information to automatically generate the conversion functions between enum
attributes and LLVM API enums in the two-way conversion between the LLVM
dialect and LLVM IR proper.

Differential Revision: https://reviews.llvm.org/D73468