Work with a gdb-remote target that doesn't handle the

qWatchpointSupportInfo packet correctly.  

In GDBRemoteCommunicationClient::GetWatchpointSupportInfo,
if the response to qWatchpointSupportInfo does not
include the 'num' field, then we did not get an answer
we understood, mark this target as not supporting that
packet.

In Target.cpp, rename the very confusingly named
CheckIfWatchpointsExhausted to CheckIfWatchpointsSupported,
and check the error status returned by 
Process::GetWatchpointSupportInfo.  If we cannot determine
what the number of supported watchpoints are, assume that
they will work.  We'll handle the failure
later when we try to create/enable the watchpoint if the
Z2 packet isn't supported.

Add a gdb_remote_client test case.

<rdar://problem/42621432> 

llvm-svn: 346561