Fix wrong refid in Doxygen's XML generator

Given the following Doxygen input:

```
/**
 * This struct must be used with f()
 */
struct t {
  int x;
}

/**
 * @param tx A struct t pointer
 */
void f(struct t *tx) {
  (void)tx;
}
```

The `f()` in struct t's comment generates a <ref> element with `refid`
equal to `some_prefix_compoundid_anchorid`, wheres the `id` in the
refered element ends up with an `id` equal to `some_prefix_anchorid`.
The anchorid here is just the compoundid prefix by "_1", so the `refid`
actually has this information duplicated.

This is a band-aid solution that tries to detect the issue and remove
the offending part from the `refid`, so that resulting links work.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>

breathe/renderer/sphinxrenderer.py

@@ -351,11 +351,29 @@ def set_context(self, context: RenderContext) -> None:
         if self.context.domain == '':
             self.context.domain = self.get_domain()
 
+    # When Doxygen's XML generator finds a reference in a comment, it
+    # generates a new <ref> element where the refid gets both a compoundid
+    # and an anchorid, where: anchorid = "_1" + compoundid. The ids that are
+    # linked to, only get the anchorid (or "_1" + compoundid!). This function
+    # tries to patch the refid, if it matches the wrong generation, by
+    # removing the compoundid, so that those refs can be correctly resolved
+    # by Sphinx.
+    def _strip_doxygen_anchorid(self, refid: str) -> str:
+        if refid:
+            parts = refid.rsplit("_", 1)
+            if len(parts) == 2 and parts[1].startswith("1"):
+                anchorid = parts[1][1:]
+                if anchorid in parts[0]:
+                    pos = parts[0].find(anchorid)
+                    return parts[0][:pos] + parts[1]
+        return refid
+
     def get_refid(self, refid: str) -> str:
         if self.app.config.breathe_use_project_refids:  # type: ignore
-            return "%s%s" % (self.project_info.name(), refid)
+            return "%s%s" % (self.project_info.name(),
+                             self._strip_doxygen_anchorid(refid))
         else:
-            return refid
+            return self._strip_doxygen_anchorid(refid)
 
     def get_domain(self) -> str:
         """Returns the domain for the current node."""