Use more consistent names

Reviewing another change I noticed that we use "getSymbols" to mean
different things in different files. Depending on the file it can
return

ArrayRef<StringRef>
ArrayRef<SymbolBody*>
ArrayRef<Symbol*>
ArrayRef<Elf_Sym>

With this change it always returns an ArrayRef<SymbolBody*>. The other
functions are renamed getELFsyms() and getSymbolNames().

Note that we cannot return ArrayRef<Symbol*> instead of
ArreyRef<SymbolBody*> because local symbols have a SymbolBody but not
a Symbol.

llvm-svn: 309840