[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.

lib/Serialization/Deserialization.cpp

@@ -7384,6 +7384,14 @@ llvm::Error ModuleFile::consumeExpectedError(llvm::Error &&error) {
 }
 
 void ModuleFile::diagnoseAndConsumeError(llvm::Error error) const {
+  auto &ctx = getContext();
+  if (ctx.LangOpts.EnableModuleRecoveryRemarks) {
+    error = diagnoseModularizationError(std::move(error),
+                                        DiagnosticBehavior::Remark);
+    // If error was already diagnosed it was also consumed.
+    if (!error)
+      return;
+  }
 
   consumeError(std::move(error));
 }