Fixed redefinition warnings with LLVM_ENABLE_MODULES

Summary:
It seems we both have the HAVE_LIBCOMPRESSION define in the config header
and in the source files definitions of some files. This causes that the
Config.h header emits the following warning when we compile the Host module:

```
In file included from <module-includes>:21:
In file included from /Users/teemperor/llvm/llvm/tools/lldb/include/lldb/Host/MainLoop.h:13:
tools/lldb/include/lldb/Host/Config.h:33:9: warning: 'HAVE_LIBCOMPRESSION' macro redefined [-Wmacro-redefined]
        ^
<command line>:1:9: note: previous definition is here
        ^
```

It's not really clear why the define is in both places (the commit message
just says it fixes some unspecified bug), but we can easily work around this
by just guarding our define in Config.h.

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D48977

llvm-svn: 336377