libstdc++: Fix incorrect size calculation in PMR resource (PR 94906)

Calculating the size of a chunk being returned to the upstream allocator
was done with a 32-bit type, so it wrapped if the chunk was 4GB or
larger.

I don't know how to test this without allocating 4GB, so there's no test
in the testsuite. It has been tested manually of course.

Backport from mainline
2020-05-04  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/94906
	* src/c++17/memory_resource.cc
	(monotonic_buffer_resource::_Chunk::release): Use size_t for shift
	operands.

libstdc++-v3/ChangeLog

 2020-05-12  Jonathan Wakely  <jwakely@redhat.com>
 
 	Backport from mainline
+	2020-05-04  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/94906
+	* src/c++17/memory_resource.cc
+	(monotonic_buffer_resource::_Chunk::release): Use size_t for shift
+	operands.
+
+	Backport from mainline
 	2020-05-03  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/94933

libstdc++-v3/src/c++17/memory_resource.cc

@@ -228,8 +228,8 @@ namespace pmr
 	  if (__ch->_M_canary != (__ch->_M_size | __ch->_M_align))
 	    return; // buffer overflow detected!
 
-	  size_t __size = (1u << __ch->_M_size);
-	  size_t __align = (1u << __ch->_M_align);
+	  size_t __size = (size_t)1 << __ch->_M_size;
+	  size_t __align = (size_t)1 << __ch->_M_align;
 	  void* __start = (char*)(__ch + 1) - __size;
 	  __r->deallocate(__start, __size, __align);
 	}