ObjectFileELF: Add support for gnu-style compressed sections

With this style, a compressed section is indicated by a "z" in the section
name, instead of a section header flag. This patch consists of two small tweaks:
- use an llvm Decompressor method in order to properly detect compressed sections
- make sure we recognise .zdebug_info (and friends) when classifying section types.

llvm-svn: 365654