validates the "self," "this," and "_cmd" pointers
that get passed into expressions.  It used to check
them aggressively for validity before allowing the
expression to run as an object method; now, this
functionality is gated by a bool and off by default.

Now the default is that when LLDB is stopped in a
method of a class, code entered using "expr" will
always masquerade as an instance method.  If for
some reason "self," "this," or "_cmd" is unavailable
it will be reported as NULL.  This may cause the
expression to crash if it relies on those pointers,
but for example getting the addresses of ivars will
now work as the user would expect.

llvm-svn: 146465