Commit a work-in-progress system runtime for Mac OS X which won't

do anything right now.  Add a few new methods to the Thread base
class which HistoryThread needs.  I think I updated all the 
CMakeLists files correctly for the new plugin.

llvm-svn: 194756

lldb/source/Target/Process.cpp

@@ -2354,6 +2354,7 @@ Process::DisableSoftwareBreakpoint (BreakpointSite *bp_site)
 size_t
 Process::ReadMemory (addr_t addr, void *buf, size_t size, Error &error)
 {
+    error.Clear();
     if (!GetDisableMemoryCache())
     {        
 #if defined (VERIFY_MEMORY_READS)