Allow C++ nested brace-list-entries to be better indented.

Allow C++ nested brace-list-entries to be better indented.

This fixes bug #24431.  The key change of this bug fix is correctly analyzing
nested brace lists when the opening element stands on the same line as both
its introductory brace and an enclosing parameter list parenthesis.

* list/progmodes/cc-align.el (c-lineup-under-anchor): New line-up function.

* list/progmodes/cc-engine.el (c-looking-at-or-maybe-in-bracelist): Accept the
presence of exactly an identifier between an open parenthesis and an open
brace as evidence of the brace starting a brace list.
(c-looking-at-statement-block): New function, extracted from
c-looking-at-inexpr-block.  Enhance it to analyze inner blocks recursively
when needed.
(c-looking-at-inexpr-block): Extract new function (see above) and call it.
(c-add-stmt-syntax): Enhance, with new &optional parameter, to supply the
prime syntactic symbol with a fixed anchor point.  When this is used, restrict
all added syntactic symbols to those having an anchor point on the same line.
Add, in addition to the current additional symbols, c-brace-list-entry when
needed; use c-looking-at-statement-block to determine the latter.
(c-guess-basic-syntax, CASE 9D): Use c-add-stmt-syntax rather than just
c-add-syntax, to assemble the syntactic context of a 'brace-list-entry, thus
getting, possibly, several accompanying syntactic entries.

* lisp/progmodes/cc-styles.el (c-style-alist, "gnu" style): New entry for
'brace-list-intro, namely c-lineup-arglist-intro-after-paren.

* lisp/progmodes/cc-vars.el (c-offsets-alist): Change the factory default
offset for 'brace-list-entry from 0 to c-lineup-under-anchor.

* doc/misc/cc-mode.texi (Syntactic Symbols): Amend the definition of
brace-list-intro.
(Brace List Symbols): Amend the example to show the new analysis of brace
lists when the first element comes on the same line as the opening brace.
(Misc Line-Up): Document the new line-up function c-lineup-under-anchor.