The existing fix-it printer can lead to difficult-to-read output
when fix-it hints are near each other.  For example, in a recent
patch to add fix-it hints to the C++ frontend's -Wold-style-cast,
e.g. for:

  foo *f = (foo *)ptr->field;
                       ^~~~~

the fix-it hints:
 replace the open paren with "const_cast<"
 replace the close paren with "> ("
 insert ")" after the "ptr->field"

would be printed in this odd-looking way:

  foo *f = (foo *)ptr->field;
                       ^~~~~
           -
           const_cast<
                 -
                 > (        )

class rich_location consolidates adjacent fix-it hints, which helps
somewhat, but the underlying problem is that the existing printer
simply walks through the list of hints printing them, starting newlines
as necessary.

This patch reimplements fix-it printing by introducing a planning
stage: a new class line_corrections "plans" how to print the
fix-it hints affecting a line, generating a vec of "correction"
instances.  Hints that are sufficiently close to each other are
consolidated at this stage.

This leads to the much more reasonable output for the above case:

  foo *f = (foo *)ptr->field;
                       ^~~~~
           -----------------
           const_cast<foo *> (ptr->field);

where the 3 hints are consolidated into one "correction" at printing.

gcc/ChangeLog:
	* diagnostic-show-locus.c (struct column_range): New struct.
	(get_affected_columns): New function.
	(get_printed_columns): New function.
	(struct correction): New struct.
	(correction::ensure_capacity): New function.
	(correction::ensure_terminated): New function.
	(struct line_corrections): New struct.
	(line_corrections::~line_corrections): New dtor.
	(line_corrections::add_hint): New function.
	(layout::print_trailing_fixits): Reimplement in terms of the new
	classes.
	(selftest::test_overlapped_fixit_printing): New function.
	(selftest::diagnostic_show_locus_c_tests): Call it.

From-SVN: r247548