[CIR][Codegen][Bugfix] use record layout to generate index for a field (llvm#270)

This is a minor fix similar to the one introduced in llvm#263.
Basically, all calls to the `buildLValueForFieldInitialization` are even
with the origin codegen `emitLValueForFieldInitialization` calls, i.e.
the field index is calculated from the record layout, but not from the
decl `field->getFieldIndex()`.

Added just one test, because looks like we need to implement some `NYI`
features first to test another places e.g. in `CIRGenExprAgg.cpp`,
though I could miss something.

Anyway, given the original codegen doesn't use `getFieldIndex` in these
places, we also should not.

All the remaining usages of `getFieldIndex` are ok.

Author: gitoleg

clang/lib/CIR/CodeGen/CIRGenExprAgg.cpp

@@ -1586,9 +1586,8 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
     if (curInitIndex == NumInitElements && Dest.isZeroed() &&
         CGF.getTypes().isZeroInitializable(ExprToVisit->getType()))
       break;
-
     LValue LV = CGF.buildLValueForFieldInitialization(
-        DestLV, field, field->getName(), field->getFieldIndex());
+        DestLV, field, field->getName());
     // We never generate write-barries for initialized fields.
     assert(!UnimplementedFeature::setNonGC());