DebugInfo: Remove 'inlinedAt:' field from MDLocalVariable

Remove 'inlinedAt:' from MDLocalVariable.  Besides saving some memory
(variables with it seem to be single largest `Metadata` contributer to
memory usage right now in -g -flto builds), this stops optimization and
backend passes from having to change local variables.

The 'inlinedAt:' field was used by the backend in two ways:

 1. To tell the backend whether and into what a variable was inlined.
 2. To create a unique id for each inlined variable.

Instead, rely on the 'inlinedAt:' field of the intrinsic's `!dbg`
attachment, and change the DWARF backend to use a typedef called
`InlinedVariable` which is `std::pair<MDLocalVariable*, MDLocation*>`.
This `DebugLoc` is already passed reliably through the backend (as
verified by r234021).

This commit removes the check from r234021, but I added a new check
(that will survive) in r235048, and changed the `DIBuilder` API in
r235041 to require a `!dbg` attachment whose 'scope:` is in the same
`MDSubprogram` as the variable's.

If this breaks your out-of-tree testcases, perhaps the script I used
(mdlocalvariable-drop-inlinedat.sh) will help; I'll attach it to PR22778
in a moment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235050 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineInstr.cpp

@@ -1712,7 +1712,7 @@ void MachineInstr::print(raw_ostream &OS, bool SkipOpers) const {
     if (!HaveSemi) OS << ";";
     DIVariable DV = cast<MDLocalVariable>(getOperand(e - 2).getMetadata());
     OS << " line no:" <<  DV->getLine();
-    if (auto *InlinedAt = DV->getInlinedAt()) {
+    if (auto *InlinedAt = debugLoc->getInlinedAt()) {
       DebugLoc InlinedAtDL(InlinedAt);
       if (InlinedAtDL && MF) {
         OS << " inlined @[ ";