[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