[ELF] Avoid referencing SectionBase::repl after ICF

It is fairly easy to forget SectionBase::repl after ICF.
Let ICF rewrite a Defined symbol's `section` field to avoid references to
SectionBase::repl in subsequent passes. This slightly improves the --icf=none
performance due to less indirection (maybe for --icf={safe,all} as well if most
symbols are Defined).

With this change, there is only one reference to `repl` (--gdb-index D89751).
We can undo f4fb5fd7 (`Move Repl to SectionBase.`)
but move `repl` to `InputSection` instead.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D116093