[lld] [LLD][COFF][NFC] Store impSym as DefinedImportData in ImportFile. (PR #107162)

[Jacek Caban via llvm-commits]

cjacek wrote:

This avoids a cast in `ImportFile::parse`, I plan to use it more for ARM64EC aux IAT handling.

https://github.com/llvm/llvm-project/pull/107162