[mach-o] Pass vectors by reference and name empty vector.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223527 91177308-0d34-0410-b5e6-96231b3b80d8
Add BumpPtrAllocator to lld::File.  Switch SimpleDefinedAtom to allocate
its SimpleRefernces using the BumpPtrAllocator.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223528 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] Switch MachOFile and MachODylibFile to use BumpPtr in lld::File

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223529 91177308-0d34-0410-b5e6-96231b3b80d8
[mach-o] fix leak in atoms -> normalized

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223530 91177308-0d34-0410-b5e6-96231b3b80d8