Return appropriate indentation for current line as Lisp code.
In usual case returns an integer: the column to indent to.
If the value is nil, that means don't change the indentation
because the line starts inside a string.
The value can also be a list of the form (COLUMN CONTAINING-SEXP-START).
This means that following lines at the same level of indentation
should not necessarily be indented the same as this line.
Then COLUMN is the column to indent to, and CONTAINING-SEXP-START
is the buffer position of the start of the containing expression.