Fix two leaks found by LSan.

A CursorPlatformAvailability can have several "unavailable" attributes, don't
leak all but the first.  I'm not sure if there can be several "deprecate"ds too,
but add the same logic there to keep the two code paths looking the same.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207076 91177308-0d34-0410-b5e6-96231b3b80d8