[MLIR] Mark additional builtin attr methods const

* Mark the following methods const:
  * `ArrayAttr::getAsRange`
  * `ArrayAttr::getAsValueRange`
  * `DictionaryAttr::getAs`
* Make `DictionarAttr::getAs` generic over the name class, such that
  `Identifier` and `StringRef` arguments get forwarded to the underlying
  call to `get`. (Made generic to avoid introducing a dependency on
  `include/mlir/IR/Identifier.h` as per the diff discussion.)

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D103822