[ELF] - Cleanup error reporting code and cover with the test. NFC.

We have the following code that is uncovered with the test:
https://github.com/llvm-mirror/lld/blob/master/ELF/Target.cpp#L95

This patch:
1) Removes "!IS" check. Because at that point of execution
(we are reolving the relocations during writing output)
we should only have InputSection type of the sections in the vector.
(because we already converted MergeInputSection in mergeSections()
and combined EhInputSections in combineEhFrameSections()).

2) Covers the "!IS->getParent()" with the test.

llvm-svn: 336106