[analyzer] Teach constraint managers about unsigned comparisons.

In C, comparisons between signed and unsigned numbers are always done in
unsigned-space. Thus, we should know that "i >= 0U" is always true, even
if 'i' is signed. Similarly, "u >= 0" is also always true, even though '0'
is signed.

Part of <rdar://problem/13239003> (false positives related to std::vector)

llvm-svn: 177806