[WebAssembly] Don't create MSSymbolWasm object for non-symbols

`__linear_memory` and `__indirect_function_table` are both generated
as imports in wasm object files but are actually symbols and don't
appear in any symbols table or relocation entry.  Indeed we
don't have any symbol type to meaningfully represent either of them.

Differential Revision: https://reviews.llvm.org/D58487

llvm-svn: 354599