[ClangImporter] Always sort top-level value lookups from Clang.

The order of lookups affects the order of imports, which can sadly end up
causing different behavior down the line.

The sort is by Clang TU source order, which isn't wonderful but should be
consistent. There are generally fewer decls without source locations that
come from Clang.

Swift SVN r28578

Author: jrose-apple

lib/ClangImporter/ClangImporter.cpp

@@ -1929,6 +1929,22 @@ void ClangImporter::lookupValue(Identifier name, VisibleDeclConsumer &consumer){
   bool FoundType = false;
   bool FoundAny = false;
   auto processResults = [&](clang::LookupResult &result) {
+    SmallVector<const clang::NamedDecl *, 16> sortedResults{result.begin(),
+                                                            result.end()};
+    const clang::SourceManager &srcMgr = pp.getSourceManager();
+    std::sort(sortedResults.begin(), sortedResults.end(),
+              [&](const clang::NamedDecl *lhs,
+                  const clang::NamedDecl *rhs) -> bool {
+      clang::SourceLocation lhsLoc = lhs->getLocStart();
+      clang::SourceLocation lhsExpLoc = srcMgr.getExpansionLoc(lhsLoc);
+      clang::SourceLocation rhsLoc = rhs->getLocStart();
+      clang::SourceLocation rhsExpLoc = srcMgr.getExpansionLoc(rhsLoc);
+      if (lhsExpLoc == rhsExpLoc)
+        return srcMgr.isBeforeInTranslationUnit(srcMgr.getSpellingLoc(lhsLoc),
+                                                srcMgr.getSpellingLoc(rhsLoc));
+      return srcMgr.isBeforeInTranslationUnit(lhsExpLoc, rhsExpLoc);
+    });
+
     // FIXME: Filter based on access path? C++ access control?
     for (auto decl : result) {
       if (auto swiftDecl = Impl.importDeclReal(decl->getUnderlyingDecl())) {