Introduce value_at_non_lval

Introduce value_at_non_lval

In some cases, while a value might be read from memory, gdb should not
record the value as being equivalent to that memory.

In Ada, the inferior call code will call ada_convert_actual -- and
here, if the argument is already in memory, that address will simply
be reused.  However, for a call like "f(g())", the result of "g" might
be on the stack and thus overwritten by the call to "f".

This patch introduces a new function that is like value_at but that
ensures that the result is non-lvalue.