[ELF] - ICF: add one more test case.

This test case check that ICF does not merge 2 sections which relocations
efer to symbols that live in sections of the different types
(regular input section and mergeable input sections in this case).

It covers the following line of code, which was uncovered previously:
https://github.com/llvm-mirror/lld/blob/master/ELF/ICF.cpp#L271

llvm-svn: 335475