[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>.
[analyzer] Translate "a != b" to "(b - a) != 0" in the constraint manager.

Canonicalizing these two forms allows us to better model containers like
std::vector, which use "m_start != m_finish" to implement empty() but
"m_finish - m_start" to implement size(). The analyzer should have a
consistent interpretation of these two symbolic expressions, even though
it's not properly reasoning about either one yet.

The other unfortunate thing is that while the size() expression will only
ever be written "m_finish - m_start", the comparison may be written
"m_finish == m_start" or "m_start == m_finish". Right now the analyzer does
not attempt to canonicalize those two expressions, since it doesn't know
which length expression to pick. Doing this correctly will probably require
implementing unary minus as a new SymExpr kind (<rdar://problem/12351075>).

For now, the analyzer inverts the order of arguments in the comparison to
build the subtraction, on the assumption that "begin() != end()" is
written more often than "end() != begin()". This is purely speculation.

<rdar://problem/13239003>
Add reverseComparisonOp and negateComparisonOp to BinaryOperator.

...and adopt them in the analyzer.
[analyzer] Also transform "a < b" to "(b - a) > 0" in the constraint manager.

We can support the full range of comparison operations between two locations
by canonicalizing them as subtraction, as in the previous commit.

This won't work (well) if either location includes an offset, or (again)
if the comparisons are not consistent about which region comes first.

<rdar://problem/13239003>
[analyzer] CmpRuns.py: Accept single files as input.

This allows us to compare two direct invocations of the analyzer on a
single source file without having to wrap the output plists in their
own directories.
[analyzer] Loc-Loc operations (subtraction or comparison) produce a NonLoc.

For two concrete locations, we were producing another concrete location and
then casting it to an integer. We should just create a nonloc::ConcreteInt
to begin with.

No functionality change.
[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)