Support `else:` in conditional declarations

```d
version (A) {
	void foo() {}
} else:
void bar() {}
void baz() {}
```
Else branches in conditional declarations can have multiple declarations. Libdparse cannot parse `else:`, and the AST node of `ConditionalDeclaration` is defined as
```d
...
Declaration[] trueDeclarations;
Declaration falseDeclaration;
...
```

So I fixed the parser, and changed that node definition to:
```d
...
Declaration[] trueDeclarations;
Declaration[] falseDeclarations;
...
```

Heh, `else:` is used exactly one place in druntime/phobos, [Vlad just added it a couple months back](https://github.com/D-Programming-Language/phobos/pull/2505/files#diff-e764cb80bf315d416f11d5ddcfdf3a94R60).  His use seems completely gratuitous to me, could just be `else version(Windows)` and put everything afterwards inside a pair of braces.