[Frontend] Add -Rmodule-interface-rebuild

Passing this will turn on remarks if the module loader needed to fall back and compile a module from an interface.

Author: Harlan Haskins

include/swift/Frontend/FrontendOptions.h

@@ -272,6 +272,10 @@ class FrontendOptions {
   /// times) when compiling a module interface?
   bool SerializeModuleInterfaceDependencyHashes = false;
 
+  /// Should we warn if an imported module needed to be rebuilt from a
+  /// module interface file?
+  bool RemarkOnRebuildFromModuleInterface = false;
+
   /// The different modes for validating TBD against the LLVM IR.
   enum class TBDValidationMode {
     Default,        ///< Do the default validation for the current platform.

include/swift/Option/FrontendOptions.td

@@ -388,6 +388,9 @@ def warn_long_expression_type_checking : Separate<["-"], "warn-long-expression-t
 def warn_long_expression_type_checking_EQ : Joined<["-"], "warn-long-expression-type-checking=">,
   Alias<warn_long_expression_type_checking>;
 
+def Rmodule_interface_rebuild : Flag<["-"], "Rmodule-interface-rebuild">,
+  HelpText<"Emits a remark if an imported module needs to be re-compiled from its module interface">;
+
 def solver_expression_time_threshold_EQ : Joined<["-"], "solver-expression-time-threshold=">;
 
 def solver_disable_shrink :

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -83,6 +83,9 @@ bool ArgsToFrontendOptionsConverter::convert(
   Opts.SerializeModuleInterfaceDependencyHashes |=
     Args.hasArg(OPT_serialize_module_interface_dependency_hashes);
 
+  Opts.RemarkOnRebuildFromModuleInterface |=
+    Args.hasArg(OPT_Rmodule_interface_rebuild);
+
   computePrintStatsOptions();
   computeDebugTimeOptions();
   computeTBDOptions();