codegen: bitcast to expanded struct type when accessing its fields

Cheerp, for structs, expands other structs that are in the first member
position. During this expansion, for technical reasons, it explicitly
already adds the padding to the expanded struct. So for structs like
this:

        struct foo {
                uint16_t a;
                //implicit padding of 2 bytes
                uint32_t b;
        };

        struct bar {
                struct foo f;
        }

It will actually become something like this:

        struct foo {
                uint16_t a;
                //implicit padding of 2 bytes
                uint32_t b;
        };

        struct bar {
                uint16_t a;
                uint8_t _padding[2];
                uint32_t b;
        }

However, accesses to the base struct's fields were not aware of this
extra field being added, and falsely assuming that, for `bar`, `b` would
still be in the second position like it is for `foo`.

The linear memory layout of these two types is, however, still exactly
the same. The only difference for LLVM is that the implicit padding is
already there.

Fixed this by bitcasting to the expanded struct type before trying to
access its fields.

Closes: leaningtech/cheerp-meta#118

clang/lib/CodeGen/CGExprConstant.cpp

@@ -2025,6 +2025,8 @@ class ConstantLValueEmitter : public ConstStmtVisitor<ConstantLValueEmitter,
           int32_t fieldIndex = cgLayout.getLLVMFieldNo(FD);
           if(fieldIndex == -1) {
             // Collapsed struct
+            C = llvm::ConstantExpr::getBitCast(C, CGM.getTypes().ConvertTypeForMem(CurType)->getPointerTo());
+            CElementType = CGM.getTypes().ConvertTypeForMem(CurType);
             continue;
           }
           Indexes.push_back(llvm::ConstantInt::get(CGM.Int32Ty, fieldIndex));