[ELF] Fix GCC8 warnings about "fall through", NFCI

[ELF] Fix GCC8 warnings about "fall through", NFCI

Add break statements in Object/ELF.cpp since the code should consider the
generic tags for Hexagon, MIPS, and PPC. Add a test (copied from llvm-readobj)
to show that this works correctly (earlier versions of this patch would have
asserted).

The warnings in X86ELFObjectWriter.cpp are actually false-positives since
the nested switch() handles all possible values and returns in all cases.
Make this explicit by adding llvm_unreachable's.

Differential Revision: https://reviews.llvm.org/D58837

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356037 91177308-0d34-0410-b5e6-96231b3b80d8