borrowck: Avoid overloading issues on 32bit architectures

On architectures where `size_t` is `unsigned int`, such as 32bit x86,
we encounter an issue with `PlaceId` and `FreeRegion` being aliases to
the same types. This poses an issue for overloading functions for these
two types, such as `push_subset` in that case. This commit renames one
of these `push_subset` functions to avoid the issue, but this should be
fixed with a newtype pattern for these two types.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-fact-collector.h (points): Rename
	`push_subset(PlaceId, PlaceId)` to `push_subset_place(PlaceId, PlaceId)`

gcc/rust/checks/errors/borrowck/rust-bir-fact-collector.h

@@ -334,7 +334,7 @@ class FactCollector : public Visitor
 		expr.get_rhs () - 1, current_bb, current_stmt);
 
     issue_read_move (expr.get_rhs ());
-    push_subset (lhs, expr.get_rhs ());
+    push_place_subset (lhs, expr.get_rhs ());
   }
 
   void visit (const CallExpr &expr) override
@@ -662,7 +662,7 @@ class FactCollector : public Visitor
       }
   }
 
-  void push_subset (PlaceId lhs, PlaceId rhs)
+  void push_place_subset (PlaceId lhs, PlaceId rhs)
   {
     auto &lhs_place = place_db[lhs];
     auto &rhs_place = place_db[rhs];