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

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.