IR: Give 'DI' prefix to debug info metadata

Finish off PR23080 by renaming the debug info IR constructs from `MD*`
to `DI*`.  The last of the `DIDescriptor` classes were deleted in
r235356, and the last of the related typedefs removed in r235413, so
this has all baked for about a week.

Note: If you have out-of-tree code (like a frontend), I recommend that
you get everything compiling and tests passing with the *previous*
commit before updating to this one.  It'll be easier to keep track of
what code is using the `DIDescriptor` hierarchy and what you've already
updated, and I think you're extremely unlikely to insert bugs.  YMMV of
course.

Back to *this* commit: I did this using the rename-md-di-nodes.sh
upgrade script I've attached to PR23080 (both code and testcases) and
filtered through clang-format-diff.py.  I edited the tests for
test/Assembler/invalid-generic-debug-node-*.ll by hand since the columns
were off-by-three.  It should work on your out-of-tree testcases (and
code, if you've followed the advice in the previous paragraph).

Some of the tests are in badly named files now (e.g.,
test/Assembler/invalid-mdcompositetype-missing-tag.ll should be
'dicompositetype'); I'll come back and move the files in a follow-up
commit.

llvm-svn: 236120
diff --git a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
index 83dab6d..546c8f3 100644
--- a/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
+++ b/llvm/lib/Transforms/IPO/DeadArgumentElimination.cpp
@@ -127,7 +127,7 @@
     // As the code generation for module is finished (and DIBuilder is
     // finalized) we assume that subprogram descriptors won't be changed, and
     // they are stored in map for short duration anyway.
-    DenseMap<const Function *, MDSubprogram *> FunctionDIs;
+    DenseMap<const Function *, DISubprogram *> FunctionDIs;
 
   protected:
     // DAH uses this to specify a different ID.
@@ -303,7 +303,7 @@
   // Patch the pointer to LLVM function in debug info descriptor.
   auto DI = FunctionDIs.find(&Fn);
   if (DI != FunctionDIs.end()) {
-    MDSubprogram *SP = DI->second;
+    DISubprogram *SP = DI->second;
     SP->replaceFunction(NF);
     // Ensure the map is updated so it can be reused on non-varargs argument
     // eliminations of the same function.