[lldb] [gdb-remote client] Refactor handling qSupported

[lldb] [gdb-remote client] Refactor handling qSupported

Refactor the qSupported handler to split the reply into an array,
and identify features within the array rather than searching the string
for partial matches.  While at it, use StringRef.split() to process
the compression list instead of reinventing the wheel.

Switch the arguments to MaybeEnableCompression() to use an ArrayRef
of StringRefs to simplify parameter passing from GetRemoteQSupported().

Differential Revision: https://reviews.llvm.org/D100146