[lld][WebAssembly] Rename SymbolTable::getSymbols to match ELF backend. NFC

The ELF backend originally used `getSymbols()` but went though a
sequence of changes that resulted in this method being called
`symbols()`.

d8f8abbd replaced `getSymbols()` with
`forEachSymbol`.

a2fc9644 replaced `forEachSymbol` with
`llvm::iterator_range`.

e9262edf replaced `llvm::iterator_range`
with `symbols()`.

Differential Revision: https://reviews.llvm.org/D131284