Refactor HAVE_LIBCOMPRESSION and related code in GDBRemoteCommunication

Summary:
The field `m_decompression_scratch_type` is only used when `HAVE_LIBCOMPRESSION` is
defined, which caused a warning which I fixed in rLLDB350675 by just marking the variable as always used.

This patch fixes this in a better way by only defining the variable (and the related `m_decompression_scratch`
variable) when `HAVE_LIBCOMPRESSION` is defined. This also required changing the way we handle
`HAVE_LIBCOMPRESSION` works, as this was previously always defined on macOS within the source file
but not in the header. Now it's always defined from within our config header when CMake defines it or when
we are on macOS.

The field initialization was moved to the header to prevent that we have `#ifdef` within our initializer list.

Reviewers: #lldb, jasonmolenda, sgraenitz, labath

Reviewed By: labath

Subscribers: labath, beanz, mgorny, lldb-commits, dblaikie

Differential Revision: https://reviews.llvm.org/D57011

llvm-svn: 352175