Replace the existing C++ implementation of extractInlinableText with
a new implementation based on swift-syntax. It uses SwiftIfConfig to
remove inactive regions (with a special mode), and a new compiler-only
entrypoint in the library to remove comments and `#sourceLocation`.
* [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
