gdb: check styled status of source cache entries

gdb: check styled status of source cache entries

Currently GDB's source cache doesn't track whether the entries within
the cache are styled or not.  This is pretty much fine, the assumption
is that any time we are fetching source code, we do so in order to
print it to the terminal, so where possible we always want styling
applied, and if styling is not applied, then it is because that file
cannot be styled for some reason.

Changes to 'set style enabled' cause the source cache to be flushed,
so future calls to fetch source code will regenerate the cache entries
with styling enabled or not as appropriate.

But this all assumes that styling is either on or off, and that
switching between these two states isn't done very often.

However, the Python API allows for individual commands to be executed
with styling turned off via gdb.execute().  See commit:

  commit e5348a7ab3f11f4c096ee4ebcdb9eb2663337357
  Date:   Thu Feb 13 15:39:31 2025 +0000

      gdb/python: new styling argument to gdb.execute

Currently the source cache doesn't handle this case. Consider this:

  (gdb) list main
  ... snip, styled source code displayed here ...
  (gdb) python gdb.execute("list main", True, False, False)
  ... snip, styled source code is still shown here ...

In the second case, the final `False` passed to gdb.execute() is
asking for unstyled output.

The problem is that, `get_source_lines` calls `ensure` to prime the
cache for the file in question, then `extract_lines` just pulls the
lines of interest from the cached contents.

In `ensure`, if there is a cache entry for the desired filename, then
that is considered good enough.  There is no consideration about
whether the cache entry is styled or not.

This commit aims to fix this, after this commit, the `ensure` function
will make sure that the cache entry used by `get_source_lines` is
styled correctly.

I think there are two approaches I could take:

  1. Allow multiple cache entries for a single file, a styled, and
     non-styled entry.  The `ensure` function would then place the
     correct cache entry into the last position so that
     `get_source_lines` would use the correct entry, or

  2. Have `ensure` recalculate entries if the required styling mode is
     different to the styling mode of the current entry.

Approach #1 is better if we are rapidly switching between styling
modes, while #2 might be better if we want to keep more files in the
cache and we only rarely switch styling modes.

In the end I chose approach #2, but the good thing is that the changes
are all contained within the `ensure` function.  If in the future we
wanted to change to strategy #1, this could be done transparently to
the rest of GDB.

So after this commit, the `ensure` function checks if styling is
currently possible or not.  If it is not, and the current entry is
styled, then the current entry only is dropped from the cache, and a
new, unstyled entry is created.  Likewise, if the current entry is
non-styled, but styling is required, we drop one entry and
recalculate.

With this change in place, I have updated set_style_enabled (in
cli/cli-style.c) so the source cache is no longer flushed when the
style settings are changed, the source cache will automatically handle
changes to the style settings now.

This problem was discovered in PR gdb/32676.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32676

Approved-By: Tom Tromey <tom@tromey.com>