getMetaDataID returns "-1" for unknown/unhandled

[blipper]

Bug description

getMetaDataID() returns "-1". This is not in compliance with the https://llvm.org/docs/LangRef.html#identifiers which requires numeric ids to be unsigned. This mostly matters for @zero_value = constant i2 0, align 4 | ID: -1

stol will fail since it expects unsigned

Expected result: Either "" or max unsigned int