DebugInfo: Metadata constructs now start with DI*

LLVM r236120 renamed debug info IR constructs to use a `DI` prefix, now
that the `DIDescriptor` hierarchy has been gone for about a week.  This
commit was generated using the rename-md-di-nodes.sh upgrade script
attached to PR23080, followed by running clang-format-diff.py on the
`lib/` portion of the patch.

llvm-svn: 236121
diff --git a/clang/test/CodeGenCXX/debug-info-flex-member.cpp b/clang/test/CodeGenCXX/debug-info-flex-member.cpp
index 25dcaab..afc9d25 100644
--- a/clang/test/CodeGenCXX/debug-info-flex-member.cpp
+++ b/clang/test/CodeGenCXX/debug-info-flex-member.cpp
@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -emit-llvm -g -triple x86_64-apple-darwin %s -o - | FileCheck %s
 
-// CHECK: !MDSubrange(count: -1)
+// CHECK: !DISubrange(count: -1)
 
 struct StructName {
   int member[];