[WebAssembly] Refactor symbol and symbol table to remove WasmSymbol references

This WasmSymbol types comes directly from the input objects
but we want to be able to represent synthetic symbols too.

This is more in line with how the ELF linker represents symbols.

This change also removes the logic from Symbol::getVirtualAddress
for finding the global address and instead has the InputFile
provide this.

Differential Revision: https://reviews.llvm.org/D41426

llvm-svn: 322145