[mlir][linalg] Replace LinalgOps.h and LinalgTypes.h by a single header.

After removing the range type, Linalg does not define any type. The revision thus consolidates the LinalgOps.h and LinalgTypes.h into a single Linalg.h header. Additionally, LinalgTypes.cpp is renamed to LinalgDialect.cpp to follow the convention adopted by other dialects such as the tensor dialect.

Depends On D115727

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D115728