[mlir] Fix two AttributeParser aborts

Reproducers that resulted in triggering the following asserts

mlir::NamedAttribute::NamedAttribute(mlir::StringAttr, mlir::Attribute)
mlir/lib/IR/Attributes.cpp:29:3
consumeToken
mlir/lib/Parser/Parser.h:126

Differential Revision: https://reviews.llvm.org/D122240