swift234

[ClangImporter] Use #line when reporting module build errors.

...or at least the same machinery as #line. We add a (referential) mirror
of the Clang buffer to Swift's source manager, then remap that particular
line to whatever Clang currently thinks is the presumed location for that
line. (This means we respect Clang line directives too.)

This also modifies Swift's source manager to be more forgiving about adding
the same virtual file range twice; it will tell you when you try to do it.

<rdar://problem/16596800>

Swift SVN r20572