Define SDBM key methods in its own cpp file.

Summary:
* Follows the convention of the tablegen-generated dialects.
* Ensures that vague linkage rules place the definitions in the dialect's object files.
* Allows code that uses RTTI to include MLIR headers (compiled without RTTI) without
  type_info link errors.

Reviewers: rriddle

Reviewed By: rriddle

Subscribers: mgorny, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, grosul1, frgossen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78039