Use the opened type from the callee declaration to open up references to
generic function builders that contain type parameters. This allows general
use of generic function builders.
Turn the generic CustomAttrTypeRequest into a helper function and
introduce a FunctionBuilderTypeRequest that starts from a ParamDecl.
This has better caching characteristics and also means we only need to
do a single cache lookup in order to resolve the type in the normal path.
It also means we don't need as much parameterization in the cache.
In addition, check that the parameter has function type in the request,
not just when late-checking the attribute, and add a check that it isn't
an autoclosure.
