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

[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()`.

d8f8abbd4a2823f223bd7bc56445541fb221b512 replaced `getSymbols()` with
`forEachSymbol`.

a2fc96441788fba1e4709d63677f34ed8e321dae replaced `forEachSymbol` with
`llvm::iterator_range`.

e9262edf0d11a907763098d8e101219ccd9c43e9 replaced `llvm::iterator_range`
with `symbols()`.

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