vim: Fix indentation at global scope after non-semantic ([{/)]}

If an unbalanced [ exists in a string or comment, this should not be
considered when calculating the indent at the top level.

Similarly, when testing for ({/}) to see if we're at the top level to
begin with, strings and comments should be skipped.
auto merge of #13925 : kballard/rust/vim_indent_bracket_fix, r=thestinger

If an unbalanced [ exists in a string or comment, this should not be
considered when calculating the indent at the top level.

Similarly, when testing for ({/}) to see if we're at the top level to
begin with, strings and comments should be skipped.