Fix invalid code that Clang trunk will soon diagnose.
There is an ambiguity between ::SizeClassMap (the typedef declared near
the start of this file) and sanitizer::SizeClassMap (found by the
'using namespace sanitizer;' near the start of this file).
Historically a Clang bug has meant that the error was not diagnosed, but
soon Clang will start diagnosing it. Explicitly qualify this use of
SizeClassMap so that it finds __sanitizer::SizeClassMap rather than
being ill-formed due to ambiguity.