Remove 'z' in .zdebug when decompressing a section.

When decompressing a compressed debug section, we drop SHF_COMPRESSED
flag but we didn't drop "z" in ".zdebug" section name. This patch does
that for consistency.

This change also fixes the issue that .zdebug_gnu_pubnames are not
dropped when we are creating a .gdb_index section.

llvm-svn: 324949