Move Repl to SectionBase.

It is currently in InputSectionBase. Only InputSections are used in
ICF, so Repl should be move to InputSection to clear the class
hierarchy or, like this patch does, to SectionBase for convenience.

The convenience of having it on the base class is that we can just
access the replacement without having to first check if it is an
InputSection. It is a bit less code and a bit faster as some of this
code is very hot.

I got up to 1.77% improvement in clang-gdb-index and no regressions
according to lnt.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320654 91177308-0d34-0410-b5e6-96231b3b80d8