[analyzer] Don't track autorelease pools created by +new.

This matches our behavior for autorelease pools created by +alloc. Some
people like to create autorelease pools in one method and release them
somewhere else.

If you want safe autorelease pool semantics, use the new ARC-compatible
syntax: @autoreleasepool { ... }

<rdar://problem/13121353>

llvm-svn: 174096