[ELF] - Add classof() member for ARMExidxSentinelSection.

Or code uses constructions like isa<ARMExidxSentinelSection>:
https://github.com/llvm-mirror/lld/blob/master/ELF/Writer.cpp#L1428
https://github.com/llvm-mirror/lld/blob/master/ELF/SyntheticSections.cpp#L2944

That is confusing, because without ARMExidxSentinelSection::classof()
these lines are equal to isa<SyntheticSection> and the code does not really do
the same what it expected to. I found no good way to break it though, but it is not nice.

Patch adds ARMExidxSentinelSection::classof().

llvm-svn: 336813