[COFF] Simplify relocation to discarded section diagnostic code, NFC
[COFF] Simplify relocation to discarded section diagnostic code, NFC

Move it out of the loop that applies relocations for readability.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@346777 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld