Switch the type field in DIVariable and DIGlobalVariable over to DITypeRefs.

This allows us to catch more opportunities for ODR-based type uniquing
during LTO.
Paired commit with CFE which updates some testcases to verify the new
DIBuilder behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204106 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SourceLevelDebugging.rst

@@ -567,7 +567,7 @@ Local variables
     metadata, ;; Reference to file where defined
     i32,      ;; 24 bit - Line number where defined
               ;; 8 bit - Argument number. 1 indicates 1st argument.
-    metadata, ;; Type descriptor
+    metadata, ;; Reference to the type descriptor
     i32,      ;; flags
     metadata  ;; (optional) Reference to inline location
   }