[analyzer] Specialize "loop executed 0 times" for for-in and for-range loops.

The path note that says "Loop body executed 0 times" has been changed to
"Loop body skipped when range is empty" for C++11 for-range loops, and to
"Loop body skipped when collection is empty" for Objective-C for-in loops.

Part of <rdar://problem/14992886>
[analyzer] Track whether an ObjC for-in loop had zero iterations.

An Objective-C for-in loop will have zero iterations if the collection is
empty. Previously, we could only detect this case if the program asked for
the collection's -count /before/ the for-in loop. Now, the analyzer
distinguishes for-in loops that had zero iterations from those with at
least one, and can use this information to constrain the result of calling
-count after the loop.

In order to make this actually useful, teach the checker that methods on
NSArray, NSDictionary, and the other immutable collection classes don't
change the count.

<rdar://problem/14992886>
[analyzer] Add IdenticalExprChecker, to find copy-pasted code.

This syntactic checker looks for expressions on both sides of comparison
operators that are structurally the same. As a special case, the
floating-point idiom "x != x" for "isnan(x)" is left alone.

Currently this only checks comparison operators, but in the future we could
extend this to include logical operators or chained if-conditionals.

Checker by Per Viberg!