Handle Swift versions unsupported by XCBuild (#7499)

XCBuild has its own notion of supported Swift versions which can differ
from the ones SwiftPM's own build system and the used compiler support.
Using a version outside of the supported range prevents building, so
we'll try to obtain the list of supported versions and intersect it with
the versions supported by a given package to find the best version
supported by both.

Author: neonichu