Adds a macro definition `MLIR_USE_FALLBACK_TYPE_IDS`. When this is
defined, the `MLIR_{DECLARE,DEFINE}_EXPLICIT_TYPE_ID` functions
explicitly fall back to string comparison.
This is useful for complex shared library setups
where it may be difficult to agree on a source of truth for specific
type ID resolution. As long as there is a single resolution for
`registerImplicitTypeID`, all type IDs can be reference a shared
registration. This way types which are logically shared across multiple
DSOs can have the same type ID, even if their definitions are
duplicated.
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