[mlir] ExecutionEngine: fix assertion on the error path

MLIR ExecutionEngine and derived tools (e.g., mlir-cpu-runner) would trigger an
assertion inside ORC JIT while ExecutionEngine is being destructed after a
failed linking due to a missing function definition. The reason for this is the
JIT lookup that may return an Error referring to strings stored internally by
the JIT. If the Error outlives the ExecutionEngine, it would want have a
dangling reference, which is currently caught by an assertion inside JIT thanks
to hand-rolled reference counting. Rewrap the error message into a string
before returning.

Differential Revision: https://reviews.llvm.org/D75508