Previously, a #if of the form:
```swift
#if (
false
)
print("x")
#endif
```
Would be emitted after #if-stripping as
```swift
false
)
print("x")
```
Because the old logic assumed conditions will always appear on one line.
Instead, for active clauses that have conditions, skip to the next line
after the end of the condition instead.
* [InterfaceGen] Remove #ifs from default args
This patch removes all #if configs form the bodies of default arguments,
which can contain multiline closures, while preserving the bodies of the
clauses that are active.
This code is generalized and should "just work" for inlinable function
bodies, which will come in a later patch.
* Address review comments
* Fix and test CharSourceRange.overlaps
* Fix CharSourceRange::print to respect half-open ranges
