[WebAssembly] Honor WASM_SYMBOL_EXPORT symbol flag

This flag means that symbol should be exported in the final binary.

The reason for this change is to allow source level annotations to
trigger a given symbol to be exported:
https://github.com/emscripten-core/emscripten/issues/7702

Differential Revision: https://reviews.llvm.org/D57869

llvm-svn: 353364