[Frontend] Add flag to use hash-based dependencies

Since prebuilt modules are going to use hashes for their dependencies, we need a flag to turn that behavior on. By default, we use modification times.

Author: Harlan Haskins

include/swift/Frontend/FrontendOptions.h

@@ -267,6 +267,10 @@ class FrontendOptions {
   /// dependencies as well.
   bool TrackSystemDeps = false;
 
+  /// Should we serialize the hashes of dependencies (vs. the modification
+  /// times) when compiling a parseable module interface?
+  bool SerializeParseableModuleInterfaceDependencyHashes = 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

@@ -64,6 +64,10 @@ def emit_interface_path
   : Separate<["-"], "emit-interface-path">,
     Alias<emit_parseable_module_interface_path>;
 
+def serialize_parseable_module_interface_dependency_hashes
+  : Flag<["-"], "serialize-parseable-module-interface-dependency-hashes">,
+    Flags<[HelpHidden]>;
+
 def tbd_install_name
   : Separate<["-"], "tbd-install_name">, MetaVarName<"<path>">,
     HelpText<"The install_name to use in an emitted TBD file">;

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -76,6 +76,9 @@ bool ArgsToFrontendOptionsConverter::convert(
 
   Opts.TrackSystemDeps |= Args.hasArg(OPT_track_system_dependencies);
 
+  Opts.SerializeParseableModuleInterfaceDependencyHashes |=
+    Args.hasArg(OPT_serialize_parseable_module_interface_dependency_hashes);
+
   computePrintStatsOptions();
   computeDebugTimeOptions();
   computeTBDOptions();

lib/Frontend/ParseableInterfaceSupport.cpp

@@ -480,8 +480,10 @@ bool ParseableInterfaceModuleLoader::buildSwiftModuleFromSwiftInterface(
     SerializationOpts.OutputPath = OutPathStr.c_str();
     SerializationOpts.ModuleLinkName = FEOpts.ModuleLinkName;
     SmallVector<FileDependency, 16> Deps;
-    if (collectDepsForSerialization(FS, SubInstance, InPath, ModuleCachePath,
-                                    Deps, Diags, DiagLoc, OuterTracker)) {
+    if (collectDepsForSerialization(
+          FS, SubInstance, InPath, ModuleCachePath, Deps,
+          FEOpts.SerializeParseableModuleInterfaceDependencyHashes,
+          Diags, DiagLoc, OuterTracker)) {
       SubError = true;
       return;
     }