Verifier: Check accessors of MDLocation

Check accessors of `MDLocation`, and change them to `cast<>` down to the
right types.  Also add type-safe factory functions.

All the callers that handle broken code need to use the new versions of
the accessors (`getRawScope()` instead of `getScope()`) that still
return `Metadata*`.  This is also necessary for things like
`MDNodeKeyImpl<MDLocation>` (in LLVMContextImpl.h) that need to unique
the nodes when their operands might still be forward references of the
wrong type.

In the `Value` hierarchy, consumers that handle broken code use
`getOperand()` directly.  However, debug info nodes have a ton of
operands, and their order (even their existence) isn't stable yet.  It's
safer and more maintainable to add an explicit "raw" accessor on the
class itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233322 91177308-0d34-0410-b5e6-96231b3b80d8

lib/AsmParser/LLParser.cpp

@@ -3348,8 +3348,8 @@ bool LLParser::ParseMDLocation(MDNode *&Result, bool IsDistinct) {
   PARSE_MD_FIELDS();
 #undef VISIT_MD_FIELDS
 
-  auto get = (IsDistinct ? MDLocation::getDistinct : MDLocation::get);
-  Result = get(Context, line.Val, column.Val, scope.Val, inlinedAt.Val);
+  Result = GET_OR_DISTINCT(
+      MDLocation, (Context, line.Val, column.Val, scope.Val, inlinedAt.Val));
   return false;
 }