Protect the operator/precedencegroup lookup code against modules that

import themselves.

It's not clear to me whether this situation is really intended, but
this does fix build modules with this strange situation.

lib/AST/Module.cpp

@@ -1008,8 +1008,13 @@ lookupOperatorDeclForName(const FileUnit &File, SourceLoc Loc, Identifier Name,
   // Record whether they come from re-exported modules.
   // FIXME: We ought to prefer operators elsewhere in this module before we
   // check imports.
+  auto ownModule = SF.getParentModule();
   ImportedOperatorsMap<OP_DECL> importedOperators;
   for (auto &imported : SourceFile::Impl::getImportsForSourceFile(SF)) {
+    // Protect against source files that contrive to import their own modules.
+    if (imported.first.second == ownModule)
+      continue;
+
     bool isExported =
         imported.second.contains(SourceFile::ImportFlags::Exported);
     if (!includePrivate && !isExported)

test/ClangModules/overlay_with_submodule.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -emit-module %s -sdk %S/Inputs -module-name HasSubmodule  -I %S/Inputs/custom-modules -o %t
+
+// REQUIRES: objc_interop
+
+@_exported import HasSubmodule
+@_exported import HasSubmodule.Submodule
+
+func useAnOperator() -> Int {
+  var x : Int
+  x = 1 + 2           // Forces a lookup of precedence groups
+  return x
+}