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/CodeGen/AsmPrinter/DwarfExpression.h b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
index 3b77a44..78ec937 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
@@ -93,13 +93,13 @@
   /// \param PieceOffsetInBits If this is one piece out of a fragmented
   /// location, this is the offset of the piece inside the entire variable.
   /// \return false if no DWARF register exists for MachineReg.
-  bool AddMachineRegExpression(const MDExpression *Expr, unsigned MachineReg,
+  bool AddMachineRegExpression(const DIExpression *Expr, unsigned MachineReg,
                                unsigned PieceOffsetInBits = 0);
   /// Emit a the operations remaining the DIExpressionIterator I.
   /// \param PieceOffsetInBits If this is one piece out of a fragmented
   /// location, this is the offset of the piece inside the entire variable.
-  void AddExpression(MDExpression::expr_op_iterator I,
-                     MDExpression::expr_op_iterator E,
+  void AddExpression(DIExpression::expr_op_iterator I,
+                     DIExpression::expr_op_iterator E,
                      unsigned PieceOffsetInBits = 0);
 };