[WebAssebmly] Remove use of Optional to fix assertion in gcc

This was causing GCC builds with fail with:
Symbols.h:240:3: error: static assertion failed: Symbol types must be
trivially destructible
  static_assert(std::is_trivially_destructible<T>(

The reason this is a gcc-only failure is that OptionalStorage has
as specialization for POD types that isn't built under GCC.

Differential Revision: https://reviews.llvm.org/D43317

llvm-svn: 325185