cindent handles the following case incorrectly:
impl X {
b: int,
//
c: int,
}
if you try and insert a new line after the `c` declaration.
To fix this, fix the get_line_trimmed() function to work properly, and
then extend GetRustIndent to keep searching backwards until it finds a
non-blank line after trimming. This lets it handle the trailing comma
case properly, as if the comment were never there.
Fixes #14041.