convenience: do shared borrow in same line as a linear pass

[tjhance]

It would be nice to be able to do this:

datatype X = X(x: int)
method bar(x: int, linear a: X)
method foo(linear a: X)
{
  bar(a.x, a);
}

Right now we have to do this instead:

datatype X = X(x: int)
method bar(x: int, linear a: X)
method foo(linear a: X)
{
  var x := a.x;
  bar(x, a);
}

Note that fixing this requires being cognizant of evaluation order, for example, if we do bar(some_linear_function(a), a.x) then either: (i) this should be disallowed or (ii) the compiler must make sure a.x is evaluated before some_linear_function(a).