[ModuleInterface] Emit error when we fail to find a dependency.

Author: graydon

include/swift/AST/DiagnosticsFrontend.def

@@ -267,6 +267,9 @@ ERROR(error_extracting_version_from_parseable_interface,none,
       "error extracting version from parseable module interface", ())
 ERROR(error_extracting_flags_from_parseable_interface,none,
       "error extracting flags from parseable module interface", ())
+ERROR(missing_dependency_of_parseable_module_interface,none,
+      "missing dependency '%0' of parseable module interface '%1': %2",
+      (StringRef, StringRef, StringRef))
 
 #ifndef DIAG_NO_UNDEF
 # if defined(DIAG)

lib/Frontend/ParseableInterfaceSupport.cpp

@@ -266,6 +266,9 @@ static bool buildSwiftModuleFromSwiftInterface(
     for (auto const &Dep : DepNames) {
       auto DepStatus = FS.status(Dep);
       if (!DepStatus) {
+        Diags.diagnose(SourceLoc(),
+                       diag::missing_dependency_of_parseable_module_interface,
+                       Dep, InPath, DepStatus.getError().message());
         SubError = true;
         return;
       }