Get the type's definition in CompInfo::from_ty

In llvm/llvm-project#147835, Clang's AST changed
so that a TagType's decl will not necessarily refer to the type's
definition, but to the exact declaration which produced the type.

For example, in

  typedef struct S T;
  struct S {
    int x;
  };

the 'struct S' type would refer to the incomplete type decl in the
typedef, causing CompInfo to fail to see the type's definition.

This patch inserts a call to use the definition when available. It fixes
the original test case in
#3275 and most of the
test failures caused by the Clang change but not all.

Author: zmodem

bindgen/ir/comp.rs

@@ -1269,6 +1269,12 @@ impl CompInfo {
         );
 
         let mut cursor = ty.declaration();
+
+        // If there is a definition, that's what we want.
+        if let Some(def) = cursor.definition() {
+            cursor = def;
+        }
+
         let mut kind = Self::kind_from_cursor(&cursor);
         if kind.is_err() {
             if let Some(location) = location {