Add a minimal fix for PR36878.

When looking for the output section and the output offset the
expectation was that the caller had looked at Repl. That works fine
for InputSections, but in the case of MergeInputSections the caller
doesn't have the section that is actually replaced.

The original testcase was failing because getOutputSection was
returning null. The slightly extended testcase also checks that
getOffset also checks Repl.

I will send a refactoring separetelly.

llvm-svn: 328332