Use range-based for loop. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205594 91177308-0d34-0410-b5e6-96231b3b80d8
SymbolTable::size() returns an unsigned int.

It's better to use the same type rather than a fixed width integer type
that may be different from the return type.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205597 91177308-0d34-0410-b5e6-96231b3b80d8