[clang][HeaderSearch] Make sure there are no backslashes in suggestedPath

Reviewers: sammccall

Differential Revision: https://reviews.llvm.org/D60995

llvm-svn: 359075
diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
index e0ffb87..a26df9a 100644
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1720,5 +1720,5 @@
 
   if (IsSystem)
     *IsSystem = BestPrefixLength ? BestSearchDir >= SystemDirIdx : false;
-  return File.drop_front(BestPrefixLength);
+  return path::convert_to_slash(File.drop_front(BestPrefixLength));
 }