Initial support for interop with class/struct/union fields. (#5849)

Add a new type, `custom_layout_type`, representing a struct type whose
size, alignment, and field offsets can be manually controlled. Use this
as the object representation type for imported C++ class types (which
also includes struct and union types), allowing us to model C++ class
type layouts. In passing, also add support for incomplete C++ class
types, mapping them into incomplete Carbon class types.

Map C++ fields into Carbon field declarations, allowing direct access to
C++ fields from Carbon. So far, no support is added for base classes nor
anonymous struct or union declarations; those will be added in
subsequent PRs. Also, we don't map C++ access control into Carbon yet,
so all C++ fields are accessible regardless of their access control.

For now we still use a `struct_type` as the object representation for
empty C++ classes, in order to continue to support our existing tests
that convert `{}` to empty C++ class types. This is temporary and should
be removed once we support interop with C++ class initialization.

toolchain/lower/function_context.cpp

@@ -218,6 +218,8 @@ auto FunctionContext::CreateAlloca(llvm::Type* type, const llvm::Twine& name)
     builder().SetCurrentDebugLocation(debug_loc);
 
     // Create an alloca for this variable in the entry block.
+    // TODO: Compute alignment of the type, which may be greater than the
+    // alignment computed by LLVM.
     alloca = builder().CreateAlloca(type, /*ArraySize=*/nullptr, name);
   }