[WebAssebmly] Add Module name to WasmSymbol

Imports in a wasm module can have custom module name.  This change
adds the module name to the WasmSymbol structure so that the linker
can preserve this module name.

This is needed to fix: https://bugs.llvm.org/show_bug.cgi?id=37168

Differential Revision: https://reviews.llvm.org/D45797

llvm-svn: 330854