[ELF] Merge verdefIndex into versionId. NFC (#72208)

The two fields are similar.

`versionId` is the Verdef index in the output file. It is set for
version script patterns and `sym@ver` symbols.

`verdefIndex` is the Verdef index of a SharedSymbol. The default value
-1 is also used to indicate that the symbol has not been matched by a
version script pattern (https://reviews.llvm.org/D65716).

It seems confusing to have two fields. Merge them so that we can
allocate one bit for #70130 (suppress --no-allow-shlib-undefined
error in the presence of a DSO definition).