[foreign] RFR 8222025: jextract generates reference to underfined type for va_list

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8222025
Webrev: https://cr.openjdk.java.net/~sundar/8222025/webrev.00/

"builtin" type is detected by source file location having "null" path. 
For each builtin record type, we generate fake Trees and add to the list 
of declarations. A builtin header file ("builtin$.h") is used as a 
target header for such declarations. This ensures that we only generate 
one copy of builitin mappings & all builtin type references resolve to 
those.

Thanks,
-Sundar