[mlir:LLVM][NFC] Cleanup attribute/enum definitions

Enum definitions are currently spread throughout the op definitions
file, making it difficult to reason about both where they are, and
where to add new ones when necessary. The attribute definitions are
in a similarish case, where while they have a dedicated .td file, there
definitions/declarations are generated in the main LLVMDialect source
files. This makes it difficult to reason about how to cleanly add new
attributes/enums.

This commit adds a dedicated LLVMEnums.td file for enum definitions,
cleans up the AttrDefs.td file, and adds a new LLVMAttrs.cpp/.h file to
home enum/attr definitions moving forward. This makes it much cleaner to
add new attributes/enums to the LLVM dialect.

Differential Revision: https://reviews.llvm.org/D136409