Reorder conditions in uses-allocator construction helper

Reorder conditions in uses-allocator construction helper

The erased_type condition is only true for code using the Library
Fundamentals TS, so assume it's less common and only check it after
checking for convertibility.

This does mean for types using erased_type the more expensive
convertibility check is done first, but such types are rare.

	* include/bits/uses_allocator.h (__is_erased_or_convertible): Reorder
	conditions. Add comments.
	* testsuite/20_util/uses_allocator/69293_neg.cc: Adjust dg-error line.
	* testsuite/20_util/uses_allocator/cons_neg.cc: Likewise.
	* testsuite/20_util/scoped_allocator/69293_neg.cc: Likewise.

From-SVN: r262945