Add llvm::object::getELFSectionTypeName().

This is motivated by https://reviews.llvm.org/D32488 where I am trying
to add printing of the section type for incompatible sections to LLD
error messages. This patch allows us to use the same code in
llvm-readobj and LLD instead of duplicating the function inside LLD.

Patch by Alexander Richardson!

llvm-svn: 301921