According to cppreference, identifiers are `(XID_Start | '_')
XID_Continue*`, which is the case as of C++23 and C2x. I have confirmed
this myself with the drafts of C++23 and C2x.

https://en.cppreference.com/w/cpp/language/identifiers

Clang indeed implements identifiers as `(XID_Start | '_') XID_Continue*`
in C++ mode and C2x mode, with a slight extension to the character set
to include some extra math characters:
231992d9b8/clang/lib/Lex/Lexer.cpp (L1517-L1530)

This also lets it bind to _any_ statement, which is necessary for some
c++ attributes that can attach to the statement in if, while, and
do/while statements.

* Add support for C23 attributes

https://en.cppreference.com/w/c/language/attributes

* Use wrapper for statements with attributes to reduce state machine size

* Missed a couple _declarators that needed attributes directly after

* Use annotated_declarator for declarators that have attributes immediately following them

* Give field and type declarators the same attribute treatment

* Add support for Microsoft-specific modifiers

Microsoft-specific modifiers are keywords can be used to modify declarators to form derived types.

The specification for the modifiers themselves is here:
https://docs.microsoft.com/en-us/cpp/cpp/microsoft-specific-modifiers?view=vs-2019

* 0.16.1

* #include can be followed by a macro name or a macro call

* In preproc_include, use preproc_call_expression

* Macro definition can contain continuation line in the argument list

* Reset grammar since line continuations are handled globally

Preserve the test.

* Parse expressions in preprocessor #if statement

* Make preproc_expression look like a normal expression

* Hide the wrapper rules
* Alias the rulres to match their corresponding expression rule
* Handle line continuations as extra tokens