[SIL] InitAccessors: Add a way to retrieve a name of the property referenced by assign_or_init instruction

This is going to be used by DI diagnostics.

(cherry picked from commit a736e18)

Author: xedin

include/swift/SIL/SILInstruction.h

@@ -4988,6 +4988,7 @@ class AssignOrInitInst
   ArrayRef<Operand> getAllOperands() const { return Operands.asArray(); }
   MutableArrayRef<Operand> getAllOperands() { return Operands.asArray(); }
 
+  StringRef getPropertyName() const;
   AccessorDecl *getReferencedInitAccessor() const;
 };
 

lib/SIL/IR/SILInstructions.cpp

@@ -1283,6 +1283,12 @@ bool AssignOrInitInst::isPropertyAlreadyInitialized(unsigned propertyIdx) {
   return Assignments.test(propertyIdx);
 }
 
+StringRef AssignOrInitInst::getPropertyName() const {
+  auto *accessor = getReferencedInitAccessor();
+  assert(accessor);
+  return cast<VarDecl>(accessor->getStorage())->getNameStr();
+}
+
 AccessorDecl *AssignOrInitInst::getReferencedInitAccessor() const {
   SILValue initRef = getInitializer();
   SILFunction *accessorFn = nullptr;