tree mapping the #inclusions that contributed to the compilation
unit, with the main source file as its root.
+ Beware --- not every source file mentioned in a compilation unit's
+ symtab structures will appear in the #inclusion tree! As of Oct
+ 2002, GCC does record the effect of #line directives in the source
+ line info, but not in macro info. This means that GDB's symtabs
+ (built from the former, among other things) may mention filenames
+ that the #inclusion tree (built from the latter) doesn't have any
+ record of. See macroscope.c:sal_macro_scope for how to accomodate
+ this.
+
It's worth noting that libcpp has a simpler way of representing all
this, which we should consider switching to. It might even be
suitable for ordinary non-macro line number info.