Diagnose modules with circular dependencies (#16075)

This can't arise from a clean build, but it can happen if you have
products lingering in a search path and then either rebuild one of
the modules in the cycle, or change the search paths.

The way this is implemented is for each module to track whether its
imports have all been resolved. If, when loading a module, one of its
dependencies hasn't resolved all of its imports yet, then we know
there's a cycle.

This doesn't produce the best diagnostics, but it's hard to get into
this state in the first place, so that's probably okay.

https://bugs.swift.org/browse/SR-7483

include/swift/Serialization/Validation.h

@@ -43,6 +43,10 @@ enum class Status {
   /// The module file is an overlay for a Clang module, which can't be found.
   MissingShadowedModule,
 
+  /// The module file depends on a module that is still being loaded, i.e.
+  /// there is a circular dependency.
+  CircularDependency,
+
   /// The module file depends on a bridging header that can't be loaded.
   FailedToLoadBridgingHeader,