Until now, only ">=" was supported in #if swift() expressions, for example:
```#if swift(>=2.1)
```#endif
This means that if we want to evaluate code only when the language version is
less than a particular version we need to do the following:
```#if !swift(>=2.1)
```#endif
An alernative to make this more readable (the "!" can be easily missed in a code
review) is to introduce another supported unary operator, "<". The previous
example could be rewritten like this:
```#if swift(<2.1)
```#endif
This commit adds support for that unary operator, along with some tests.
* Narrow allowance of 3+ components numeric literal to condition part of the
directive.
* Allow 3+ components in '#if' directive in decl list position as well.
There are two similar but separate code paths for conditionally
compiling declarations and statements. Previously, only statements were
properly covered with a diagnostic transaction.
rdar://problem/24844513
