Add support for handling absolute symbols in ELF

Most address represented in lldb as section plus offset and handling of
absolute addresses is problematic in several location because of lack
of necessary information (e.g. Target) or because of performance issues.

This CL change the way ObjectFileELF handle the absolute symbols with
creating a pseudo section for each symbol. With this change all existing
code designed to work with addresses in the form of section plus offset
will work with absolute symbols as well.

Differential revision: http://reviews.llvm.org/D17450

llvm-svn: 261859