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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236120 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/WinCodeViewLineTables.cpp

@@ -20,11 +20,11 @@ namespace llvm {
 
 StringRef WinCodeViewLineTables::getFullFilepath(const MDNode *S) {
   assert(S);
-  assert((isa<MDCompileUnit>(S) || isa<MDFile>(S) || isa<MDSubprogram>(S) ||
-          isa<MDLexicalBlockBase>(S)) &&
+  assert((isa<DICompileUnit>(S) || isa<DIFile>(S) || isa<DISubprogram>(S) ||
+          isa<DILexicalBlockBase>(S)) &&
          "Unexpected scope info");
 
-  auto *Scope = cast<MDScope>(S);
+  auto *Scope = cast<DIScope>(S);
   StringRef Dir = Scope->getDirectory(),
             Filename = Scope->getFilename();
   char *&Result = DirAndFilenameToFilepathMap[std::make_pair(Dir, Filename)];