[MLIR] Check for duplicate entries in attribute dictionary during custom parsing
- Verify that attributes parsed using a custom parser do not have duplicates.
- If there are duplicated in the attribute dictionary in the input, they get caught during the
dictionary parsing.
- This check verifies that there is no duplication between the parsed dictionary and any
attributes that might be added by the custom parser (or when the custom parsing code
adds duplicate attributes).
- Fixes https://bugs.llvm.org/show_bug.cgi?id=48025
Differential Revision: https://reviews.llvm.org/D90502