[analyzer] Use SymExprs to represent '<loc> - <loc>' and '<loc> == <loc>'.

We just treat this as opaque symbols, but even that allows us to handle
simple cases where the same condition is tested twice. This is very common
in the STL, which means that any project using the STL gets spurious errors.

Part of <rdar://problem/13239003>.

llvm-svn: 177800