[Serialization] Intro -Rmodule-recovery to remark about silent errors

Deserialization recovery silently drops errors and the affected decls.
This can lead to surprises when a function from an imported module
simply disappears without an explanation.

This commit introduces the flag -Rmodule-recovery to report as remarks
some of these previously silently dropped issues. It can be used to
debug project configuration issues.

include/swift/Basic/LangOptions.h

@@ -238,6 +238,9 @@ namespace swift {
     /// Emit a remark after loading a module.
     bool EnableModuleLoadingRemarks = false;
 
+    /// Emit remarks about contextual inconsistencies in loaded modules.
+    bool EnableModuleRecoveryRemarks = false;
+
     /// Emit a remark when indexing a system module.
     bool EnableIndexingSystemModuleRemarks = false;