Driver: add support for `-gz` and `-gz=`

These options control the behaviour of the compression of debug info
sections on ELF targets.  Our behaviour slightly diverges from the
behaviour of GCC.  `-gz` maps to the `-compress-debug-sections` rather
than `-compress-debug-sections=zlib` or
`-compress-debug-sections=zlib-gnu`.  This small divergence allows us to
be compatible across versions of binutils (=zlib support was introduced
in 2.26, while earlier versions only support =zlib-gnu).  This also
allows users to not have to worry about the version of the assembler
they may be using if they are not using the IAS.  Previously, users
would have had to go through the internal option
`-compress-debug-sectionss` and pass that through to the assembler,
which is no longer needed.

llvm-svn: 305165