Add verification for pruned methods for `emitCustomBuilder` and
`emitCheckedCustomBuilder` with proper diagnostic about shadowed methods.
Without this verification, `mlir-tblgen` with `--gen-attrdef-decls`
would segmentation fault if custom builder is provided with its body,
but if method is pruned out due to duplication with other builders.
Fixes#160227
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Co-authored-by: Justin Kim <jaewoo.kim@hyperaccel.ai>
aaf23f0887