[CIR][CodeGen][Bugfix] Generate field index with respect to layout (#263)

There is a bug in the code generation: the field index for the
`GetMemberOp` is taken from the `FieldDecl`, with no respect to the
record layout. One of the manifestation of the bug is the wrong index
generated for a field in a derived class that does not take into the
account the instance of the base class (that has index 0).

You can take a look at the example in
`test/CIR/CodeGen/derived-to-base.cpp`, i.e. the current test is not the
correct one
```
// CHECK-DAG: !ty_22C23A3ALayer22 = !cir.struct<class "C2::Layer" {!ty_22C13A3ALayer22, !cir.ptr<!ty_22C222>
// CHECK-DAG: !ty_22C33A3ALayer22 = !cir.struct<struct "C3::Layer" {!ty_22C23A3ALayer22

// CHECK: cir.func @_ZN2C35Layer10InitializeEv

// CHECK:  cir.scope {
// CHECK:    %2 = cir.base_class_addr(%1 : cir.ptr <!ty_22C33A3ALayer22>) -> cir.ptr <!ty_22C23A3ALayer22>
// CHECK:    %3 = cir.get_member %2[0] {name = "m_C1"} : !cir.ptr<!ty_22C23A3ALayer22> -> !cir.ptr<!cir.ptr<!ty_22C222>>
```
As one can see, the result (of ptr type to `!ty_22C222` ) must have the
index `1` in the corresponded struct `ty_22C23A3ALayer22`.

Basically the same is done in the `clang/CodeGen/CGExpr.cpp`, so we
don't invent something new here.

Note, this fix doesn't affect anything related to the usage of
`buildPreserveStructAccess` where the `field->getFieldIndex()` is used.

Author: gitoleg

clang/lib/CIR/CodeGen/CIRGenExpr.cpp

@@ -271,7 +271,9 @@ LValue CIRGenFunction::buildLValueForField(LValue base,
     if (!IsInPreservedAIRegion &&
         (!getDebugInfo() || !rec->hasAttr<BPFPreserveAccessIndexAttr>())) {
       llvm::StringRef fieldName = field->getName();
-      unsigned fieldIndex = field->getFieldIndex();
+      auto& layout = CGM.getTypes().getCIRGenRecordLayout(field->getParent());
+      unsigned fieldIndex = layout.getCIRFieldNo(field);
+
       if (CGM.LambdaFieldToName.count(field))
         fieldName = CGM.LambdaFieldToName[field];
       addr = buildAddrOfFieldStorage(*this, addr, field, fieldName, fieldIndex);

clang/test/CIR/CodeGen/derived-to-base.cpp

@@ -82,7 +82,7 @@ void C3::Layer::Initialize() {
 
 // CHECK:  cir.scope {
 // CHECK:    %2 = cir.base_class_addr(%1 : cir.ptr <!ty_22C33A3ALayer22>) -> cir.ptr <!ty_22C23A3ALayer22>
-// CHECK:    %3 = cir.get_member %2[0] {name = "m_C1"} : !cir.ptr<!ty_22C23A3ALayer22> -> !cir.ptr<!cir.ptr<!ty_22C222>>
+// CHECK:    %3 = cir.get_member %2[1] {name = "m_C1"} : !cir.ptr<!ty_22C23A3ALayer22> -> !cir.ptr<!cir.ptr<!ty_22C222>>
 // CHECK:    %4 = cir.load %3 : cir.ptr <!cir.ptr<!ty_22C222>>, !cir.ptr<!ty_22C222>
 // CHECK:    %5 = cir.const(#cir.ptr<null> : !cir.ptr<!ty_22C222>) : !cir.ptr<!ty_22C222>
 // CHECK:    %6 = cir.cmp(eq, %4, %5) : !cir.ptr<!ty_22C222>, !cir.bool