Add a small refinement to the qSymbol:: support in lldb.

This is a packet which allows the remote gdb stub to ask for the address
of a symbol in the process.  lldb sends the packet (offering to provide
addresses for symbol names) after every solib loaded.  I changed lldb so
that once the stub has indicated that it doesn't need any more symbol
addresses, lldb will stop sending the qSymbol:: packet on new solib loads.

This can yield a performance benefit over slower communication links when
there are many solibs involved.

<rdar://problem/23310049> 

llvm-svn: 257569