gdbhooks: Handle references to vec* in VecPrinter

vec.h has this method:

  template<typename T, typename A>
  inline T *
  vec_safe_push (vec<T, A, vl_embed> *&v, const T &obj CXX_MEM_STAT_INFO)

where v is a reference to a pointer to vec.  This matches the regex for
VecPrinter, so gdbhooks.py attempts to print it but chokes on the reference.
I see the following:

  #1  0x0000000002b84b7b in vec_safe_push<edge_def*, va_gc> (v=Traceback (most
  recent call last):
    File "$SRC/gcc/gcc/gdbhooks.py", line 486, in to_string
      return '0x%x' % intptr(self.gdbval)
    File "$SRC/gcc/gcc/gdbhooks.py", line 168, in intptr
      return long(gdbval) if sys.version_info.major == 2 else int(gdbval)
  gdb.error: Cannot convert value to long.

This patch makes VecPrinter handle such references by stripping them
(dereferencing) at the top of the relevant functions.

gcc/ChangeLog:

	* gdbhooks.py (strip_ref): New. Use it ...
	(VecPrinter.to_string): ... here,
	(VecPrinter.children): ... and here.

Author: acoplan-arm

gcc/gdbhooks.py

@@ -472,6 +472,11 @@ def get_vec_kind(val):
     else:
         assert False, f"unexpected vec kind {kind}"
 
+def strip_ref(gdbval):
+    if gdbval.type.code == gdb.TYPE_CODE_REF:
+        return gdbval.referenced_value ()
+    return gdbval
+
 class VecPrinter:
     #    -ex "up" -ex "p bb->preds"
     def __init__(self, gdbval):
@@ -483,10 +488,10 @@ def display_hint (self):
     def to_string (self):
         # A trivial implementation; prettyprinting the contents is done
         # by gdb calling the "children" method below.
-        return '0x%x' % intptr(self.gdbval)
+        return '0x%x' % intptr(strip_ref(self.gdbval))
 
     def children (self):
-        val = self.gdbval
+        val = strip_ref(self.gdbval)
         if intptr(val) != 0 and get_vec_kind(val) == VEC_KIND_PTR:
             val = val['m_vec']