[lld][ELF] Rename SymbolTable::symbols() to SymbolTable::getSymbols(). NFC

This change renames this method match its original name and the name
used in the wasm linker.

Back in d8f8abbd the ELF SymbolTable
method `getSymbols()` was replaced with `forEachSymbol`.

Then in a2fc9644 `forEachSymbol` was
replaced with a `llvm::iterator_range`.

Then in e9262edf we came full circle
and the `llvm::iterator_range` was replaced with a `symbols()` accessor
that was identical the original `getSymbols()`.

`getSymbols` also matches the name used elsewhere in the ELF linker as
well as in both COFF and wasm backend (e.g. `InputFiles.h` and
`SyntheticSections.h`)

Differential Revision: https://reviews.llvm.org/D130787