Enable empty base alignment on Environment to make HV32 build on Windows

Summary:
In Callable.h, we assert the invariant that the offset of
`parentEnvironment_` and `size_` in `Environment` is the same as
`parentEnvironment` and `size` in `SHEnvironment`, respectively.
However, this is not always true with MSVC (Clang with enforce the
same layout on Windows).

`Environment` inherits from `VariableSizeRuntimeCell` and
`TrailingObjects<Environment, GCHermesValue>` (with this order).
With HV32 release mode, `VariableSizeRuntimeCell` has 4 bytes,
`TrailingObjects<Environment, GCHermesValue>` has 0 bytes and has an
`alignas(8)` specifier.

The default memory layout is:
0:   base class VariableSizeRuntimeCell
4:   padding
8:   parentEnvironment_ (if casting to TrailingObjects, the address is here)
12:  size_
16:  trailing objects of GCHermesValue

The reason is that MSVC does remove the empty class's storage (so
`TrailingObjects` occupies 0 byte), but it still enforces that the
empty base class still has a unique address. And due to the
`alignas(8)` specifier, it has to add 4 padding bytes to make the
address aligned to 8 bytes.

With the `__declspec(empty_bases)` attribute, the memory layout is:
0:  base class VariableSizeRuntimeCell
4:  parentEnvironment_
8:  size_
12: padding
16: trailing objects of GCHermesValue

This is the same as SHEnvironment.

Reviewed By: tmikov

Differential Revision: D87686988

fbshipit-source-id: 014b2aeda05750c78e5d8a160249c43ce91ccf3f

Author: lavenzg

include/hermes/VM/Callable.h

@@ -24,7 +24,24 @@ namespace hermes {
 namespace vm {
 
 /// Environment storing all escaping variables for a function.
-class Environment final
+/// We need the `HERMES_EMPTY_BASES` attribute here because without it, MSVC
+/// or Clang on Windows will add 4 padding bytes before parentEnvironment_ under
+/// HV32 release mode (TrailingObjects<Environment, GCHermesValue> has
+/// an alignas(8) specifier), which causes different layout than SHEnvironment.
+/// Memory layout without HERMES_EMPTY_BASES, with HV32 release mode:
+/// 0:  base class VariableSizeRuntimeCell
+/// 4:  padding
+/// 8:  parentEnvironment_
+/// 12: size_
+/// 16: trailing objects of GCHermesValue
+/// Memory layout with HERMES_EMPTY_BASES, with HV32 release mode:
+/// 0: base class VariableSizeRuntimeCell
+/// 4: parentEnvironment_
+/// 8: size_
+/// 12: padding
+/// 16: trailing objects of GChermesValue
+/// Once we upgrade to c++20, we should use [[no_unique_address]] here.
+class HERMES_EMPTY_BASES Environment final
     : public VariableSizeRuntimeCell,
       private llvh::TrailingObjects<Environment, GCHermesValue> {
   friend TrailingObjects;