[move-only] Teach SILGenApply how to emit subscripts with borrowed base values.

I also added a bunch of tests that showed the behavior of subscripts/other accessors with the following combinations of semantics:

1. get only.
2. get/set.
3. get/modify.
4. read/set.
5. read/modify.

rdar://109746476

Author: gottesmm

lib/SILGen/ArgumentSource.h

@@ -280,7 +280,7 @@ class ArgumentSource {
 
   ArgumentSource copyForDiagnostics() const;
 
-  void dump() const;
+  LLVM_DUMP_METHOD void dump() const;
   void dump(raw_ostream &os, unsigned indent = 0) const;
 
 private:

lib/SILGen/SILGenApply.cpp

@@ -3042,6 +3042,14 @@ Expr *ArgumentSource::findStorageReferenceExprForMoveOnly(
 
   auto argExpr = asKnownExpr();
 
+  // If we have a subscript, strip it off and make sure that our base is
+  // something that we can process. If we do and we succeed below, we return the
+  // subscript instead.
+  SubscriptExpr *subscriptExpr = nullptr;
+  if ((subscriptExpr = dyn_cast<SubscriptExpr>(argExpr))) {
+    argExpr = subscriptExpr->getBase();
+  }
+
   // If there's a load around the outer part of this arg expr, look past it.
   bool sawLoad = false;
   if (auto *li = dyn_cast<LoadExpr>(argExpr)) {
@@ -3097,6 +3105,12 @@ Expr *ArgumentSource::findStorageReferenceExprForMoveOnly(
   // has a move only base.
   (void)std::move(*this).asKnownExpr();
 
+  // If we saw a subscript expr and the base of the subscript expr passed our
+  // tests above, we can emit the call to the subscript directly as a borrowed
+  // lvalue. Return the subscript expr here so that we emit it appropriately.
+  if (subscriptExpr)
+    return subscriptExpr;
+
   return result.getTransitiveRoot();
 }