Split out filename & directory from DIFile to start generalizing over DIScopes

This is the first step to making all DIScopes have a common metadata prefix (so
that things (using directives, for example) that can appear in any scope can be
added to that common prefix). DIFile is itself a DIScope so the common prefix
of all DIScopes cannot be a DIFile - instead it's the raw filename/directory
name pair.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177239 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/DIBuilder.cpp b/lib/IR/DIBuilder.cpp
index 5a82ed0..0622b90 100644
--- a/lib/IR/DIBuilder.cpp
+++ b/lib/IR/DIBuilder.cpp
@@ -117,11 +117,14 @@
 /// for a file.
 DIFile DIBuilder::createFile(StringRef Filename, StringRef Directory) {
   assert(!Filename.empty() && "Unable to create file without name");
-  Value *Elts[] = {
-    GetTagConstant(VMContext, dwarf::DW_TAG_file_type),
+  Value *Pair[] {
     MDString::get(VMContext, Filename),
     MDString::get(VMContext, Directory),
   };
+  Value *Elts[] = {
+    GetTagConstant(VMContext, dwarf::DW_TAG_file_type),
+    MDNode::get(VMContext, Pair)
+  };
   return DIFile(MDNode::get(VMContext, Elts));
 }