Generate pointer-valued macro constants

[tamird]

The Clang macro fallback can evaluate macro forms that cexpr cannot
parse, but it discards pointer-valued expressions. Emit data pointer
macros as raw Rust pointer constants. Keep dependent expressions
eligible for fallback evaluation.

Materialize pointer macro types from a final fallback parse so
deferred type resolution does not retain cursors invalidated by a
later reparse.

Related to #3347.

[tamird]

cc @metaspace @ojeda

[tamird]

r? @emilio

[ojeda]

It could be nice to split in two commits, to see how the test changes -- from a quick test against mainline I see:

+pub const BEFORE_DECL: *mut later = 3u64 as usize as *mut later;
+pub const CONST_PTR: *const later = 4u64 as usize as *const later;
+pub const TYPEDEF_PTR: *mut later = 5u64 as usize as *mut later;
+pub const MAP_FAILED: *mut ::std::os::raw::c_void = 18446744073709551615u64 as usize
+    as *mut ::std::os::raw::c_void;
+pub const MAP_FAILED_ALIAS: *mut ::std::os::raw::c_void = 18446744073709551615u64
+    as usize as *mut ::std::os::raw::c_void;
 pub const MAP_FAILED_EQUALS_ITSELF: u32 = 1;
 pub const REDEFINED_FROM_INT: u32 = 1;
-pub const REDEFINED_ALIAS: u32 = 1;
+pub const REDEFINED_ALIAS: *mut ::std::os::raw::c_void = 2u64 as usize
+    as *mut ::std::os::raw::c_void;

Is that the expected change here?

I am surprised there was no other test that needed an update for the REDEFINED_ALIAS case.

(A couple surprising but pre-existing issues discussed elsewhere IIRC are the value of REDEFINED_FROM_INT and the type of e.g. MAP_FAILED_EQUALS_ITSELF)