swift234

Clang importer: feed a Clang prepprocessor through to canInferDefaultArgument.

Omitting needless words while importing the full name function
involves inference of default arguments, so we can get here while
building Swift name lookup tables. It takes a lot of machinery to
trigger this problem; tests forthcoming.

There *must* be a better way to ensure this never happens, but it
requires a bit more refactoring in the Clang importer.