FileArchive cleanup

Moved code used only by isDataSymbol from find to isDataSymbol member
function. Also changed the return type of isDataSymbol because
previously "if (isDataSymbol(...))" meant "if it is *not* a data symbol"
which is opposite from what you'd expect.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217285 91177308-0d34-0410-b5e6-96231b3b80d8