Cxx: support typeof and __typeof__ keywords of the gcc extension

[masatake]

The parser treats them the same as decltype keyword of the C++ standard.
Close #3620.

An example:

input.c:

// Derrived from glibc
# define __attribute_copy__(arg) __attribute__ ((__copy__ (arg)))
# define weak_alias(name, aliasname) _weak_alias (name, aliasname)
# define _weak_alias(name, aliasname) \
  extern __typeof (name) aliasname __attribute__ ((weak, alias (#name))) \
    __attribute_copy__ (name);

int
__brk (void *addr)
{
	/* ... */
	return 0;
}
weak_alias (__brk, brk)

cmdline: ./ctags --param-CPreProcessor._expand=true --fields-C=+'{macrodef}' --fields=+'{signature}' --kinds-C=+x -o - input.c

tags output:

__attribute_copy__	/tmp/baz.c	/^# define __attribute_copy__(/;"	d	file:	signature:(arg)	macrodef:__attribute__ ((__copy__ (arg)))
__brk	/tmp/baz.c	/^__brk (void *addr)$/;"	f	typeref:typename:int	signature:(void * addr)
_weak_alias	/tmp/baz.c	/^# define _weak_alias(/;"	d	file:	signature:(name,aliasname)	macrodef:extern __typeof (name) aliasname __attribute__ ((weak, alias (#name))) __attribute_copy__ (name);
brk	/tmp/baz.c	/^weak_alias (__brk, brk)$/;"	x	typeref:typename:__typeof(__brk)
weak_alias	/tmp/baz.c	/^# define weak_alias(name, aliasname) _weak_alias /;"	d	file:	signature:(name,aliasname)	macrodef:_weak_alias (name, aliasname)

brk is extracted well.

[codecov]

Codecov Report

Base: 82.81% // Head: 82.81% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (8a6c482) compared to base (dfdabb5).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3621   +/-   ##
=======================================
  Coverage   82.81%   82.81%           
=======================================
  Files         223      223           
  Lines       54451    54462   +11     
=======================================
+ Hits        45092    45103   +11     
  Misses       9359     9359           

Impacted Files	Coverage Δ
parsers/cxx/cxx_keyword.c	100.00% <100.00%> (ø)
parsers/cxx/cxx_parser_variable.c	91.68% <100.00%> (+0.11%)	⬆️
parsers/cxx/cxx_token_chain.c	82.41% <100.00%> (+0.14%)	⬆️

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