Thread the static build configuration (formed from language options) in
to the macro plugin handler, which will serialize it for use in the
macro implementation. test this with a simple macro that checks
whether a particular custom configuration (set via `-D`) is enabled or
not.
This required some re-layering, sinking the logic for building a
StaticBuildConfiguration from language options down into a new
swiftBasicSwift library, which sits on top of the C++ swiftBasic and
provides Swift functionality for it. That can be used by the C++
swiftAST to cache the StaticBuildConfiguration on the ASTContext,
making it available for other parts of ASTGen.
